The 28-year-old is pleased to be linking up with his former boss Paterson again and told the Official Aberdeen website "Having worked for Steve and Duncan Shearer I know their football philosophy. They've always believed in positive attacking football which I like to think suits my style of play. Having the chance to play at the top level is something no player wants to turn down so it goes without saying that I'm delighted to be here."
Boss Paterson was delighted at his 2nd capture of the week "Paul is one of the best passers of the ball in the First Division, has a great left foot and is an excellent dead ball specialist. In addition he is a proven goal scorer. He's at an age where he's hitting the peak of his career and I'm sure he'll be an excellent addition to my squad here at Pittodrie."
Like Steve Tosh, Sheerin will head to Portugal with the rest of the squad.
A fee of around £15,000 is thought to be heading to Ayr in payment for Sheerin.