Coláiste Lurgan is one of the most popular gaeltachts around. They run courses throughout the summer, and you have the option of a 2 week course or a 3 week course. The age range is usually aimed at all secondary students, but Cursa A is restricted to 1st and 2nd year students, and Cursa D is for senior students, from Transition Year up. It is usually €850 for a 3 week course, and €680 for a 2 week course.
Check out this cool "Festival of Colours" - one of the many things done at Lurgan each year.