Dialects of the Irish Language
Irish has different dialects, just as English does. Each dialect has minor differences in grammar, pronunciation, and vocabulary. Roughly speaking, Ulster and Munster are two ends of a spectrum, with Connacht somewhere in the middle.
Which dialect should you learn? If you're in a class, your best bet is to focus on the dialect used by your teacher. Once you get comfortable with the basics of the language, you can change dialects without much difficulty. If you're learning on your own, you might want to pick Connemara (Connacht) Irish because it follows the "standard" a little more closely than the other dialects. Or simply pick the book that best suits your learning style, and start with whatever dialect it teaches. Dialect isn't as much of an issue as you might think, because any book designed for beginners will stick fairly close to the standard, even if that book focuses on one particular dialect.
There is a standard for Irish grammar, called An Caighdeán Oifigiúl. There is no official standard for pronunciation. Except where noted, in this guide I will follow the standard for grammar, and give you the most widespread pronunciations. If you learned a different way of pronouncing certain words, stick with the pronunciation you know.