As Lughaidh said, all the dialects are equally important. I believe (and I may be mistaken) that there are more speakers of the Connacht dialect than of Munster or Ulster, but it really doesn't matter hugely. If you feel a special connection to one part of Ireland or another, that might be the dialect you'd want to focus on. Or perhaps you find you like the sound of one more than the other. Or maybe you find a teacher or local speakers who have specialized in one or the other, making it easier on you to specialize.

Just remember that they're just dialects, not different languages. If you speak one, others will understand you. Most learners learn a mixture of dialects at first and specialize later if they feel so inclined.

Redwolf