Ireland would still be British. My estimates have the island with between 6 and 24 million residents.
As well it's colonies would have seen a 75-25 split the other way. IE the US would not be 25% Catholic, it would be 75%.
Tensions in Ireland were not cause by the religious difference in the slightest, although religion became a convenient way to delineate who fell on which side and eventually a source of bigoted stereotypes and slurs.
Can't agree with the strident nature of this. The Hiberno Normans, while resented by native Gaelic population, eventually blended into the larger population. The old cliche was they became "more Irish than the Irish themselves". The real warfare started after the Reformation and as part of the larger European age of religious wars. Then you had the settlements of Protestants from Scotland and England to keep the Old Hiberno norman and Gaelic Chieftains in line. Next was Irish rebellion/massacres against these planters followed by Cromwells revenge. You simply can't take religion out of the equation. There was a brief period of some non conformist/Catholic unity in the United Irishmen rebellions but these were squashed rather easily. Of course, presently religious differences are now largely just a matter of fact thing rather than the cause of the division.
