"Roy is a player I know very well, having tried to get him from Cork before his move to Sunderland," United boss Craig Levein said.
"He is a very direct player, with lots of pace and is a natural and proven goalscorer.
"Although he can play wide right and midfield, I see him as a striker and I'm sure he'll be a big success in The SPL. I'm also certain that United supporters will enjoy seeing him play for the team.
"He'll benefit greatly from playing regular first team football at the highest level in Scotland and therefore this deal is not only good for Dundee United, but will also be to the advantage of the player and his club.
"I'd like to thank Sunderland AFC for their ready co-operation on this matter and, once again, it's pleasing to see that Dundee United is recognised as a club where players can be developed and improved further."