The Irish scrum-half (pictured) will combine playing for Jon Hill’s side with working at Premiership outfit Exeter Chiefs on their player development programme.
Cushion, who is about to qualify as a physiotherapist, gained coaching experience at Albion last season in the Championship. He was Plymouth’s skills and analysis player-coach.
Exmouth’s head coach Hill said: “This is a big signing for us, and one that will add experience and leadership to what is still a relatively young side.
“I have been massively impressed with Ruairi’s attitude and commitment throughout all discussions in recent weeks. He is totally honest and has been true to his word every step of the way.
“He has been given glowing references from everyone who has worked with him and coached him, and I am delighted to be able to bring him on board.”
Former club captain Cushion, who announced his decision to leave Albion on New Year’s Eve, made his Albion debut against Rotherham on September 13, 2009 and went on to make nearly 150 competitive appearances for the club. His final match in Plymouth colours came against Jersey in February, with his season ending early due to a broken thumb.
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