hmm thats not quite true...well in my experience traveling on the 16.50 everyday to Dundalk.
You queue up down the wee corrider for the enterprise like everyone else.. If you are early enough they don't particularly care if you have a "boarding Pass" or not. They do have notices stating that if you don't you are supposed to stay in a seperate queue but there are ways around that.. So far I have not seen prority boarding for Northern Ireland passengers, (mind you it is an election year?) they also let dundalk passengers who did join the seperate queue on to the train about 10mins before departure, noticeable by the running down the platform
Except on fridays or bank holiday weekends there are usually enough seats for all.. the train does empty quite a bit at Dundalk.