And why not, though I suppose staying as a Dominion might not be what the OP wanted either. As mentioned earlier by the point of the 21/22 and after that I'm not sure how you'd get Ireland to stay within the UK.
I think you have to look for some external stimulus to drive this. No matter how you rearrange the UK's set up you can't get around the fact that England grossly unbalances any attempt at a federal structure. Even then you run into the problem that by 1900 Irish people didn't, if they'd ever, see the Irish national identity see as subordinate to being British.
Even then it's quite difficult for this reason. There are plenty of plausible scenarios where Britain intervenes in Ireland. It's hard to construct these so that they would try to reintegrate Ireland rather than just installing and propping up a friendly regime.