I always assumed it referred to collar pins that pass under a tie, push it up a little and hold the collars in place. That would plausibly give it the meaning of being put in a tight spot. But afaik it's a uniquely Irish phrase, so it might be a direct translation into English. I'm meeting a friend tomorrow who teaches Irish - if nobody comes up with an answer for you, I'll ask him if he knows anything about it.
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