Just a note, the 90, 91, 92 and Luas are covered by the extension you can get on your rail ticket.
Any of the other buses you could get, you would have to pay for.
I dont find the train packed at peak times coming into town, really, only problem is if you are last to jump on the train on Heuston outbound.
The frequency of service would be more of an issue for me, rather than whether I get a seat or not, though I understand that Newbridge is the best served station on the arrow line.