Threat of invasion? No, the English invasion had happened before Cromwell's time.

What happened in 1922 was a revolution (well, to be technical, an act of armed insurrection) which partly succeeded in its aims, but which those in the north would not accept. Since then it is the republicans who have been the aggressors, and Northern Ireland has been part of the United Kingdom of Great Britain and Northern Ireland in which other UK-ers have a legitimate interest. The legitimate interest is precisely what the southern republicans have not got. Theirs, even if a war, would be a war of conquest and territorial acquisition - or of regime change in another country.