You don't mention your age or your location but, you can hardly go wrong with Buntús Cáinte. It will help you build a nice vocabulary, develop a sense of sentence formation and structure and will help you learn to pronounce the words written on the page. This last point is especially important for beginners. What you see on paper doesn't always "make sense" with how it is pronounced. I mean, it makes perfect sense in Irish but with not with our english trained brain.
There is also a great free download on the net. Its a flash card program and it pronounces the words for you. I don't have the link off hand but if you email me at I will look it up and send the link to you.