In reply to Kean:
I live in NI and get the train with a folding bike weekly into Belfast at peak times. There is one bike compartment per train, usually at the very front. There is space for maybe 4-5 bikes leant against each other. Half the time mine is the only bike there but the space has folding seats too so if you get on at a late point on the route (and at a peak time) you might find folk are sitting in the bike area. You can't book a space. I think the rules are that only folding bikes are permitted pre 9.30 but I've never seen that enforced...there are often full size bikes on board and none of the staff seem to mind. If they are just on for a couple of stops you often see people just standing with a bike in the space between carriages too.
Not taken a bike on Goldline or Enterprise...but the latter is more like GB trains where you often need to book to get a seat and it can get very busy.
My advice would be not to worry about it and turn up, but with full size bikes and kit you may want to avoid peak times (trains going into Belfast for 8-9 am and leaving at 5ish)
Hope this helps...