Argos sell a range of costumes, but prices seem a little high to me. Did a quick search on Google, and found a few places, including one that has some second-hand costumes for sale:
THE DUBLIN COSTUME COMPANY BASEMENT LEVEL 26 FITZWILLIAM STREET UPPER DUBLIN 2 IRELAND Opening Hours; Monday - Saturday 9.30am -6.00pm Open Sunday 25th Oct 2pm-6pm & Thurs 29th Oct until 8pm Ph; (353)1-6685200 Email; info@dublincostumes.com
Come down Baggot Street from Stephen's Green, and take a right turn at the cross-roads where Larry Murphy's Pub is. We are on the left hand side of the street, approaching from Baggot st. Once you find the building number 26, you will find us in the basement there.
Depending on the look you want, it might be worth trawling vintage or second-hand clothes shops to create your own costume.
Some of the girls in work got costumes @ www.thecostumeshop.ie and were very happy with them, €30 - €40 if thats not too much?? (depending on the costume of course)