Oh for fuck's sake, of course it's an offensive stereotype and shouldn't be used by anyone who cares the slightest about denigrating others.
I am Irish. Never heard the phrase at all in Ireland for the first 20-something years I lived there. Heard it for the first time from a colleague when living in the UK, in reference to an English women who had babies 10 or 11 months apart, and I was gobsmacked that anyone thought it was an acceptable phrase to use. I've heard it a handful of times in Ireland since then, usually when taking the piss out of someone (such as a cousin who announced her second pregnancy when she had a 4-month old baby).
But outside Ireland talking about someone who is not Irish, it's exactly the same form of wrongness as the phrase "working like a black", which I also heard people use in a group conversation when living in the UK. When pulled up on it by a friend, who said something very mild like "you can't really use that an expression like that", the rest of the group jumped on her with all the defensiveness and platitudes I've seen on this thread.
- It's not meant to be offensive.
- Don't be so sensitive; what a silly thing to get worked up about
- My black friend says it all the time and he thinks it's fine
- It's based on history so it's accurate
- It's a compliment really (about how hard black people work)
No, it's still a bigoted phrase used in ignorance and complacent privilege!
Irish people using the phrase "Irish twins" is a knowing, cultural in-joke - I've only encountered it being used to take the piss out of somebody, but it might be used in a more neutral way.
British, American, etc people - regardless of whether or not you have some Irish ancestor - cannot use the phrase as a cultural in-joke because they are not culturally Irish. It can only ever be othering. Just don't.