This is a very difficult AHC to achieve, since there's no one POD within the time frame that achieves this. You'd have to go far back enough that the butterflies might get rid of the United Kingdom of begin with (or any Britain-wide state)
Resentment of the English and resistance to British rule had always existed in Ireland, and that's pretty well documented by the unrest and various wars and rebellions throughout its history. Someone suggested the potato famine not happening, but that doesn't do the trick - remember that the Irish had fought Cromwell with every fiber of their being, and had rebelled again in 1798 (which created the United Kingdom).
The Irish had essentially shown that, under any circumstance, they had no interest in political association with Britain (when you consider that the colonization of Ireland was the flavor of the day for nearly the entirety of Britain's relationship with Ireland). After all, even after the Irish Free State existed as a Dominion, they revoked it some fifteen or so years later. The Irish would most certainly never agree to a federal relationship with Britain, as asked above.
You might need a POD in the 1300s. As such, this AHC is very difficult.