Ross Doneghan, 25, joins another Scottish hooker, David Cherry, at the club. As a back-rower he spent two years in South Africa playing Currie Cup for Border Bulldogs, but it was while playing for Ayr that he moved from back-row to front.
A former Scottish Schools Cup winner with Stewart's Melville College, he played for English National League Two North club Darlington Mowden Park last season and caught the eye of Scottish head coach James Buckland.
Younger brother Mike, a utility back, signed a one-year contract extension in March, having joined from Rotherham Titans.
"To get this opportunity to play and train together with Mike and to work with James Buckland is something I cannot wait to do," said Ross Doneghan. "Hooker will be a really competitive position next season and I am sure I can still learn a lot from Adam Kwasnicki and David Cherry, who have played the position longer than me."