For an Iceland that retains an Irish population, I think you'd need the Vikings to bypass it. If the Vikings really want it, they'll have it.
The first Irish settlers are not going to have a very good time. The climate is much worse than they are used to, their coping skills are not that good for the environment, and their agricultural package isn't that well-adapted. You'd be likly to see a significant time lag before they stated expanding, as they adapted to the poorer environment. Also, most people going there might be wanting to get away from something. And adapting to the local conditions would change the Celts.
Many peoples in harsh, arctic environments respond by higher levels of local cooperation and less warfare. If the Icecelts went a few steps in this direction, they might develop more of a tradition of communal response to disasters and problems.
So when the vikings arrive, they do a hang-together rather than hang separatly, and put up some coordinated and spirited resitance.
I am thinking this will make them not worth the bother to the Vikings.
In OTL they found a cold, but mainly empty land for the taking. In TTL, they find somewhere full of strongly resisting people, dirt poor and climatically unpleasant.
Conquerable, but not worth the effort. If you're going to go a-conquering, you might as well go somewhere richer and warmer.
EDIT: Come to think of it, if the first viking contacts coincided or were quickly followed by the rise of a charismatic and ambitious leader, the viking outsiders could be used as a political excuse to gather and consolidate power. Battle-victories would cement the leaders position. Which would be possible if the vast majority of vikings never cared much about the place.