""I recently heard the expression, Irish twins. What does it mean and would you cast some light as to its origin?"

[A] What it means is easy enough to explain. It refers to siblings born in the same calendar year, or otherwise less than twelve months apart. It's clearly a deeply derogatory comment about the stereotypical fecundity (and lack of contraception) of Irish Catholic families. It's probably twentieth-century, but I can find little evidence that would help to tie it down (it may be relevant that it isn't listed in the 1984 edition of Eric Partridge's Dictionary of Slang and Unconventional English). So far as I can tell from the places I've found it, all online, it's primarily an American expression; it's also known in Britain, but it doesn't often find its way into print, no doubt because it is considered offensive."

From a google of Irish Twins