The novelty factor would explain those crowds in the 40s. My dad remembers Man. U. coming over to play Belfast Celtic once a year and Celtic Park would be jointed – but the regular Irish League games would also pull big crowds.
What spelt the death-knell for LoI attendances was the advent of televised English football in the 70s, particularly going head-to-head against The Big Match on UTV on Sunday afternoons. People then had the option of watching George the postman play while standing on a terrace in Milltown or watching George Best play from the comfort of their own couch.
Nowadays Sky Sports’ Super Duper Sunday in Widevision HD is merely dancing on the grave dug by The Big Match, while an attendance that seemed to be pushing 2500 for Pats v Drogheda last night was deemed to be “a decent crowd” in the half-time chatter along the Camac. They didn’t all just come to ogle Trappatoni, it turned out to be a good game of football.