i had to buy mine from aib but i had to pay them 100 euros to do it. it was a birthday present from me to me lol so i took the plunge.
i found it quite difficult to locate anywhere else i could do it without having a uk bank account. the only online places i could find required more paperword than it would take to buy a house
i had to set up a current ac with them. it was really easy though. just a form of id and a bill to prove you are at your address. that takes a day or two to set up and then you can buy the shares.
The cheapest place I found for one off shares is www.tdwaterhouse.ie - never got them in time for last trip - I think you need them about three months before hand - so must get organised for March!