The “open-ended Irish backstop” (Brexit)

Downing Street let it be known that May’s withdrawal agreement might after all be acceptable, if only the open-ended Irish backstop could be removed. Brussels in return let it be known that Johnson’s new Whitehall sherpa, David Frost, was in town, and that the backstop was the issue under discussion.
The Guardian

This excerpt is awashed in metonymies; Downing Street refers to the appointed executive power in the UK; May is short for Theresa May, the former Prime Minister of the UK; Brussels represents the European Parliament; Johnson refers to the current Prime Minister of the UK, Boris Johnson; Whitehall stands for Parliament, a Sherpa (The Guardian spells it lowercase) is a Himalayan mountain guide; maybe that last one is a metaphor.

But what is an Irish backstop? Is that also a metonymy? A synecdoche? A euphemism? What?

The BBC has an article that explains what it means in a single sentence

The backstop is a position of last resort, to maintain a seamless border on the island of Ireland.

But until Brexit, I had never heard of a backstop being applied to an inland border. Lexico (formerly Oxford Dictionaries) defines it as

1. A thing placed at the rear of something as a barrier or support.
1.3 An emergency precaution or last resort.

Online I found an infographic that uses the term, but it's enclosed in quotes, which suggests that the meaning might be unfamiliar or ambiguous.

Why is the solution by Brussel, which is meant to preserve the free movement of goods and people within Ireland, called a backstop? Since when? Did someone came up with this baffling term? And is the Irish backstop a metonymy, a metaphor, or something else?

According the the OED, back-stop or backstop is defined as:

a. Something placed at the back to serve as a barrier; spec. a mound of earth or embankment set up behind a target on a rifle range. Also figurative.

1851 F. Starr Twenty Years Trav. Life xiii. 143 The remaining shaft..broke off short, and that which when we started was a gig, was now a back stop for horses' heels.

1904 G. F. Goodchild & C. F. Tweney Technol. & Sci. Dict. 37/1 Back stops (Cotton Spinning), buffers, used to prevent the mule carriage from going beyond a certain point during its inward run.

1946 Sports Afield Jan. 55/1 (D.A.) Put up two targets on the backstop for the second barrel practice.

1947 Landfall 1 43 This being the first issue of Landfall, there is no previous survey to provide a convenient back-stop to discussion.

1954 W. Faulkner Fable (1955) 343 The railroad embankment..would serve as a backstop for what bullets neither flesh nor wood absorbed.

This suggests that the word is used figuratively when there is no physical barrier (this entry not updated since 1973, though). Since then everyday use suggests it is losing it figurative connotation and become a more literal expression of a wide variety of situations.

The OED also reports the derivative use of backstop as a verb:

backstop v. transitive, to support, back up; to supply with necessary additional resources. colloquial (orig. and chiefly U.S.).

1977 N.Y. Times 17 June A26/2 The International Monetary Fund can backstop the private system.

Consider that backstop is a word with a definition. Backstops come in many varieties. In baseball a screen behind the catcher is a backstop; it stops the ball if the catcher fails to catch it. It’s not a metonymy or a metaphor or anything else. This particular backstop is modified by “Irish,” and thus made specific to the Brexit deal.

My neighbor can be my backstop for taking care of my kids in the event that I can’t find a babysitter. That’s not a metaphor; it’s the use of a word with a definition.

As broadcast on CSPAN-2 on September 1, 2019, from the National Archives, author Philip Mudd (Black Site) described the views of one of the terrorists (Kalid Sheikh Muhammad) on the importance of destroying the United States before destroying what he considers corrupt Arab regimes as follows: "The Americans are the backstop to these corrupt regimes."

A last resort is not figurative. Neither is backstop. It seems to me, at least, that it’s literal, however complicated the conditions that trigger it and its actual provisions may be.

See also this Ngram for backstop, which shows that in the last 60 years, the word has been used in economics (especially energy and climate control research), insurance, and finance.

A good swr for a last resort would be backstop.