Why "an" rather than "na" for plural?
For the same reason that "a herd of cows" or "a flock of birds" uses a singular group noun for multiple cows or birds.
Lucht leanúna is a bunch/squad/group/horde/crowd of followers.
I think 'fans' would be a more accurate word here than 'supporters'. I'd translate 'supporters' as 'tacadóirí'
Football teams tend to have Supporter Clubs rather than Fan Clubs.