The companies that hire them out are usually at the graduations so you just get them on the day. However, if it's for a photo or something that's not on the day of graduation I googled graduation gown hire for you and got this website: www.mcginleygowns.com (I have no association with them but I'm sure you could call them and they would answer your question!).
I rang Mcginleys and they gave me the name Phelan and Conan. I googled them and found their website. Just gave them an call and they hire out the gown/hat for €40.