I don't think thats true. I know plenty of Bohs fans who go to Dubs or Meath games - league and all. Maybe its the case in Sligo. Once had a flatmate who was a Sligo fan and he said there was a major split between Association and GAA fans in Sligo. However you have to look at the GAA's attitude towards "foreign games" as the source of this animosity. I know a few Shamrock Rovers fans who were once Fanatical Dubs fans too but won't go to games anymore because of the Thomas Davis episode and who'd blame them for that?
We've had some AGM's and Members info meetings in Na Fianna and you'd have to envy the facilities they have in comparison to what LOI sides can boast.