The Clueless soundtrack had little to do with the Bosstones' success. They had a small, strong following to begin with. Then, the 3rd Wave Ska craze hit. Clueless put them on the soundtrack because they were popular not vice versa. Then, when the ska craze was over, they went back to their small, strong following. Also, they were the Bosstones and a friend suggested the Mighty Mighty to get around the legal headache with these guys.
The Departed gave the Dropkick Murphys some exposure but they too had a pretty strong following before that.
I saw them both quite a while ago. It was in 1997 at the Riviera in Chicago for their Boston on the Road tour with Bim Skala Bim.