minors: under 18
Junior: Any age (might be minimum age of 16, not sure if GAA implemented that)
Intermediate: same as above
Senior: same as above
Basically Junior,Intermediate and Senior are classifications for "adult" teams with Senior being the "best" teams, and Junior being the "least best" teams. Intermediate is the in between. Its a bit like soccer in that teams get promoted and relegated bewteen junior,inter, and senior. Within junior,inter, and senior there can be further breakdowns like Junior A and Junior B, and Senior A and Senior B. This is usually different in each county.