Well, Mormons are a minority in Ireland right now.
If by "minority" you mean "significant minority", sure, if they get enough converts, they'll be a significant minority. That's almost a tautology.
I'm not sure about them joining the IRA, though. Granted, the Ian Paisley types would probably be pretty fired-up against them(if you thought Maryolatry was bad, wait'll you see God And His Wife Have Sex), and if the Orangemen start launching armed attacks against the LDS, the Mormons might start trying to defend themselves in some way.
That said, the default sectarianism in that place seems so entrenched, it might be unlikely that anyone would have room in their pysche for entertaining an American-based faith based on Jesus walking around the US midwest. Interesting scenario, though.