On-line Grammar Course
Gaelchultúr has an online grammar course, Gramadach gan Stró!.
The course is aimed at those who already have a reasonably good standard of spoken Irish but who wish to improve their grammar and to learn useful idiomatic and colloquial expressions.
Interactive exercises are a core part of the course and the user receives instant feedback. You can try a sample unit of the course for free here. You can register for the full course here.