Irish Gaelic: Dialects
Just as English has regional variants, so does Irish.
As an example, ask in a room full of people what they call a glass of Coca-Cola. "a Coke", "a Pop", "a soda" depending on what are you're in. Anyone who has traveled on the East coast has run into the variations of pronunciation that makes "Boston" into "Bahstun" and "harbor" into "hah-buh". We have Southern drawls and Western twangs and Midwestern flattened-vowels. We usually understand each other, though, so it's not too bad.
Irish is no different. There are three major dialects of Gaelic which are just different enough to be difficult for a beginner. It doesn't help that the major teaching aids (Irish on Your Own, Teach Yourself Irish, and Learning Irish do not share the same dialects.
The three major versions of Irish (Munster, Ulter, and Connacht) are mostly similiar, but they differ in how some words are pronounced and how some words are used.