When I was a teenager I went to both of those two - Gael Linn one in Eacleim in Belmullet and Colaiste na bhFiann in both Ros Muc and Garbally College in Ballinasloe.
Honest opinion - Gael Linn - even some of the teachers spoke English to us. Learnt nothing.
Colaiste na bhFiann - actually didn't speak a word of English and learnt a lot. However, that said, I "put up" with being there rather than actually enjoying it. Theres a lot of rubbish -ie- learning off the entire mass (including the creed) off by heart which took a week. We also were accompanied at all times (and I mean at every moment of the day except when actually in the bathroom) which I couldn't stand but stuck it out for the Irish. The lessons are good though, especially the grammar ones.
And I went from a C in the Junior Cert to an A in the Leaving Cert.
Depends on what your looking for really!