Can you elaborate that a bit, please?
I'm not an expert on Scottish history, but from some things that I've read about the period I gather that couple that pushed Christianity in Scotland more heavily towards Roman Catholicism, actively working against whatever remnants of the separate 'Celtic' church were still around in their time. In fact, that seems to have been
one of the reasons for Margaret's canonisation. (Malcolm was less pious than Margaret, but may also have been influenced by the fact that his own main centre of power was in the more 'Roman' areas of the south whereas his main rivals were based further north in areas where the 'Celtic' church was more influential.)
So, if you want an Irish-Scottish union after that time, you'll need either to undo their work -- which would involve a serious struggle in Scotland itself,
before you could move against England, and would probably see your opponents inviting English forces as allies -- or you need to convert Ireland more solidly to the Roman church which IOTL seems not to have happened untill
after Henry II's invasion. In fact, the Irish not being "good" Catholics is said to have gained papal support (and a blessed banner) for Henry... although admittedly the fact that (for the only time so far) the Pope himself was
English might have helped in that respect as well.