"Ironic" vs. "ironical"
Given these definitions:
ironic:
- containing or exemplifying irony
- ironical
- coincidental; unexpected
ironical:
- pertaining to, of the nature of, exhibiting, or characterized by irony or mockery
- using or prone to irony
The only difference I could plausibly assert is that ironical means it uses irony, where ironic means it is an example of irony. That is, this conclusion of this book is ironic. This ironical book is a good, fun read. This ironical author is one of my favorites.
Parenthetically speaking, the inclusion of ironical in the definition of ironic makes sense in this interpretation because an example of irony must by definition use irony.
Does this help you not use ironical except as an adjective?
http://ngrams.googlelabs.com/graph?content=ironic%2C+ironical&year_start=1800&year_end=2000&corpus=0&smoothing=3
Ironical http://ngrams.googlelabs.com/chart?content=ironic%2Cironical&corpus=0&smoothing=3&year_start=1800&year_end=2000
To the best of my recollection, examples I've seen in literature tend to use "ironical" when applied to speech or behavior, and "ironic" when applied to events or circumstances. (That would loosely follow the pattern of "historical" and "historic.")