What is /iə/ in English?

Take, for example, the word beer. Here we would use the transcription /bɪə/ in Southern Standard British English (SSBE). Notice that this word has two phonemes, the consonant /b/ and the vowel /ɪə/. That vowel—often referred to as the NEAR vowel—is a single vowel. We use two symbols to represent it because this vowel changes quality as we say it. It starts of with a KIT-like quality, [ɪ] and finishes with a schwa-like quality, [ə]. Notice that we don't treat it as two distinct vowels, but as a single unit, even though we use two symbols to represent it.

In English we also talk about the HAPPY vowel (the vowel at the end of the word happy). For most younger speakers this is an allophone of the FLEECE vowel which occurs in unstressed, unchecked syllables (an unchecked syllable is just a syllable with no consonant at the end of it). For other speakers this is an allophone of the KIT vowel. Because this vowel may be considered one of either /ɪ/ or /i:/ by different speakers, and because it occurs at the end of such a large number of English words, we use the convention of representing this occurrence of the vowel using a single < i > with no length marks in transcriptions of SSBE. (Whether this is more confusing than helpful, is open to some debate. John Wells remarked in one of his blogs that "it seemed like a good idea at the time"!).

Now, consider the verb copy. The normal transcription for this in SSBE would be /kɒp.i/. At the end of that transcription, we see an occurrence of /i/. Now we can add an -er suffix onto this verb to turn it into the noun copier. In speech, this suffix is realised as a single schwa, /ə/. So the transcription you'll see in many dictionaries for this word will be /ˈkɒp.i.ə/. This, for example, is the transcription given by the Cambridge English Dictionary. And also by John Wells's Longman Pronunciation Dictionary. Notice that, according to this transcription, there are two distinct phonemes at the end of this word /i/ and /ə/, not a single diphthong /ɪə/. This also fits neatly with the morphology of the word.

The use of /i/ to represent a vowel in positions where the FLEECE-KIT distinction, iː vs. ɪ, is neutralized is a relatively modern innovation, and fifty years ago, nearly all phoneticians were using /ɪə/ in words like copier. Indeed, this transcription is still used by some dictionaries. This however, gives the rather unsatisfactory result of a morpheme being represented by half of a vowel (urgh!).

^{Note: The capitalised words FLEECE, KIT, HAPPY and so forth, are the keywords from John Wells's lexical sets. They can be thought of as names for the vowels concerned (so FLEECE is used to refer to the phoneme /i:/ and so forth). The HAPPY vowel refers, of course, to the second vowel in the word happy. Wells couldn't use a one syllable word there because this vowel only occurs in unstressed, unchecked syllables—any single syllable word would inevitably be pronounced with stress in its citation form and therefore with a regular full length FLEECE vowel.}

It might make it easier to answer your question if you gave more information about what context in which you need to know these symbols (are you more interested in being able to read dictionary transcriptions correctly, or in being able to make your own transcriptions that will be considered "correct" by somebody else?). The short answer to "How different are they pronounced?" is "not very": being able to distinguish something like /iə/ from something like /ɪə/ is not an essential feature of a native-sounding English accent. If they are distinguished, it will likely be by syllable count, as Araucaria suggests: /iə/ in a modern transcription is likely to represent two vowels pronounced in two separate syllables.

The symbol /ɪə/ is often used as a fairly conservative phonemic transcription of a British English vowel sound that was traditionally described as a tautosyllabic falling centering diphthong (thus, a more explicit transcription of the traditional phonetic realization would be [ɪə̯], with the non-syllabic diacritic on the schwa). Words that were pronounced with [ɪə̯] in RP may be pronounced instead in many present-day British accents with a phonetic monophthong [ɪː]. In addition to [ɪː], some speakers may use in certain contexts a pronunciation like [ɪjə] that sounds like it has two syllables, with a vowel like the one in the word "fleece" (the FLEECE vowel) in the first and a vowel like the one in the second syllable of "comma" or "letter" in the second, and a gliding transition between them.

Because of this, it seems to be disputed whether the inventory of contemporary standard British English speakers should be analyzed as containing a tautosyllabic diphthong phoneme /ɪə/: it may be the case that we can analyze some speakers as instead having a phonemic inventory containing the long monophthong /ɪː/, the FLEECE vowel /ɪj/, and schwa /ə/ where a sequence of the FLEECE phoneme and the schwa phoneme can be phonetically realized as [ɪə] or [ɪː] instead of as [ɪjə] because of optional processes of compression. (Variation in speaker intuitions about syllable count doesn't only exist for British English speakers: American English speakers have similar variation in how many syllables they assign to words like foil and snarl).

Longer Discussion

The use of the IPA length marker <ː> in phonemic transcriptions

Some transcription systems for English do not make of the IPA letter <ɪ>. Purely "quantitative" transcriptions use /iː/ for the sound found in knee or fleece, and /i/ for the sound found in kit. You can see an overview of some different transcription systems in this post by John Wells: IPA transcription systems for English.

When you are looking at a transcription in a dictionary, you can find out if it uses a purely quantitative or a quantitative-qualitative system by looking at the key, or at the transcriptions used for words that you already know how to pronounce.

When you are making your own transcriptions, I would not recommend using a purely quantitative transcription, as they don't seem to be popular currently.

Wells favors a quantitative-qualitative system, and these seem to be pretty popular. But an important point to remember is that the use of the length marker in quantitative or qualitative-quantitative phonemic transcription systems for English is completely unrelated to the non-phonemic processes that affect phonetic vowel length in English based on stress and what type of consonants follow the vowel. The word "seat" is pronounced with a phonetically shorter vowel than the word "seed", but in a quantitative or qualitative-quantitative phonemic transcription, both are transcribed with the phoneme /iː/ with a length marker: /siːt/, /siːd/.

Also, you can't necessarily assume that a phonemically "long" vowel will be realized with greater phonetic length than a phonemically "short" vowel: for example, if I remember correctly, the low vowel /æ/, although phonemically classified as a "lax" or "short" vowel and therefore not marked with a length marker in phonemic transcriptions, tends to be realized as fairly long phonetically.

Because of the potential for confusion between phonemic and phonetic length—concepts which it is essential that an EFl learner distinguish—some phoneticians such as Jack Windsor Lewis prefer to avoid using length markers in phonemic transcriptions (see The Undesirability of length marks in EFL phonemic transcription (1975)).

The traditional RP "near" diphthong is /ɪə/ (in one syllable)

The symbol /ɪə/ is traditionally used to transcribe a vowel phoneme identified as the vowel in the word "near". Historically, this originates from a "long e" sound (which in other contexts became the vowel phoneme often transcribed /iː/) that became "colored" by the following r. In accents that have lost word-final r as a consonant, like standard British English, the remnant of this "coloring" is the reason near sounds different from knee, even though neither ends in a consonant sound.

In "Received Pronunciation" (RP), the "near" vowel is considered to only take up a single syllable, but to move from one vowel sound to another; it is therefore categorized as a diphthong, like /aɪ/ or /aʊ/. The monophthongal vowel thought to sound most similar to the first element of the RP "near" vowel is /ɪ/, as in "kit", while the monophthongal vowel thought to sound most similar to the final element of the RP "near" vowel is /ə/, as in the last syllable of "comma" or "letter". Thus, the typical transcription of the RP "near" diphthong is /ɪə/. Of course, in a purely quantitative transcription system where the "kit" vowel is written as /i/ and the "fleece" vowel is written as /iː/, the near vowel would be written /iə/ rather than /ɪə/.

/ɪə/ or /iə/ is also used for sounds that didn't originate as r-colored "long e"

As Araucaria's answer mentions, the symbols /ɪə/ and /iə/ do not only correspond to the reflex of "r-colored long e" (as I describe it) in British English.

The other main source is original sequences of two vowels in hiatus (you can usually tell from the spelling, which will involve two "vowel letters" like ia or ea). In contemporary accents, these may be pronounced in two syllables or one, depending on certain factors like stress, morphological structure, and perhaps frequency (in that more learned, less frequent words may be more likely to be pronounced with two syllables), and there is a fair amount of variation.

When there is no stress on the vowel written "ɪ" or "i", it can often be replaced with a non-syllabic glide /j/, resulting in /jə/.

When there is stress on the first vowel and it is pronounced in a separate syllable from the following "ə", it is usually written with a length marker (assuming we are using quantitative or quantitative-qualitative transcription), so you would typically see /iːə/ (or, with the syllable divisions explicitly marked, /iː.ə/).

Variation: monophthongization, dieresis and smoothing

Geoff Lindsey's blog post "The demise of ɪə as in NEAR" presents a case against using the concept of a tautosyllabic diphthong "ɪə" in descriptions of "contemporary Standard Southern British" accents. He writes:

In the earlier standard/reference accent of British English, Received Pronunciation, words like NEAR contained a centring diphthong, ɪə. This was a vowel which glided from the lax quality ɪ to the quality ə within a single syllable. It can be heard in this clip from a 1930s Pathé documentary about beef and beer: [audio clip removed]

Such a vowel is heard relatively rarely today. Although British dictionaries still use “/ɪə/” in their transcriptions, a lax diphthong of this type is now rather old-fashioned. In contemporary Standard Southern British (SSB), we hear tend to hear either

1. a long pure vowel, the monophthong ɪː; or
2. a form in which the tense FLEECE vowel is followed by schwa, which we could write as ɪjə or, with traditional symbols, as /iːə/; this form can plausibly be considered to comprise two syllables.

[...]

In SSB, NEAR is commonly ɪjə or ɪː. Some speakers use both; for them, NEAR may be considered varisyllabic, like WIRE and SOUR. Others seem to use only ɪː. Either way, the transcription of NEAR as a monosyllabic lax diphthong “/ɪə/” is now rather out of date.

More blog post about the phonetics of the "near" vowel and related sounds:

• "Smoothing, then and now" (Geoff Lindsey)

• "rising diphthongs" (John Wells)

• "diphthong, schmiphthong" (John Wells)

• "Smoothing and Compression" (Jack Windsor Lewis)

The Wells "rising diphthongs" post has this interesting bit at the end:

Inspired by Jones’s pair reindeer — windier, another phonetician (I think it was Bjørn Stålharne Andrésen, but I can’t lay my hands on the reference, so this is from memory) performed a listening experiment in which he got speakers to imagine that as well as reindeer and roedeer we also have a kind of deer called a windeer; he asked them to pronounce in suitable carrier sentences the words windeer (kind of deer, with its falling diphthong in the second syllable) and windier (more windy, with its putative rising diphthong), and then played the results to listeners who were asked to decide which of the two words had been said. They proved unable to do this with better than random success. So the distinction between NEAR (my ɪə) and happY plus schwa (my i‿ə) may indeed be ‘felt subjectively by the speaker in slow utterance’, but the hearer cannot reliably detect it.