united Ireland in the UK.
with Ireland united and in the UK it doesn't suffer the economic damage that was caused by dividing the Ireland otl. the unionists are happy and IIRC the Irish themselves were happy enough being in the UK, they just wanted home rule, until the Easter rising so there shouldn't be too many problems.
ideally the UK would be reformed into some sort of federal structure so the Irish get the home rule they want, whilst the unionists have nothing to complain about because the whole country is getting the same treatment and thus they won't feel like they're being ignored/abandoned.
united independent Ireland is my second option, again due to map tidiness and prevention of economic damage, preferably with some sort of federal structure as well to make the unionists happy (or at least less angry)