In the time I have spent in Ireland I think I am yet to buy my own drink! You will often find that when visiting a bar or pub here in Co. Wexford (or anywhere in Ireland) and people find out you are a visitor, they will buy you a drink*.
Of course you should always offer to buy them a drink in return.
If you are going out with a group in Ireland (with Irish friends), they will sometimes buy drinks in rounds, ie, one person goes to the bar and buys eveyones drinks and then the next round it is someone elses turn to buy. Although this will depend entirely on the group of people you are with, so just watch and see if people are buying their own drinks or it they're going to buy them in rounds.
*no guarantees, sorry :)