The Irish fly-half has signed for Championship side Doncaster Knights, where he will replace former Albion number 10 Paul Roberts in their squad.
Cusack, who arrived at Plymouth midway through the 2012-13 season, joins the likes of fellow regular starters from last season Iain Grieve, Neale Patrick, Brett Beukeboom, Toby Howley-Berridge, Rob Langley, Ben Woods, Tom Fidler in officially leaving Brickfields in the past month.
It had initially looked like Cusack might stay but he informed the club this week that he would be accepting a deal at Castle Park.
Albion’s director of rugby Graham Dawe, who has also seen back-up fly-half Lawrence Rayner leave, said: “Declan has been a top operator for this club, but we understand his desire to continue is rugby career at the highest level possible.”
Cusack said: “Firstly, I would like to thank everyone at Plymouth Albion RFC, it was an honour and a privilege to play and captain the club. I really enjoyed my time there.
“I am extremely excited to sign for Doncaster. As soon as I spoke to Clive (Griffiths) about the staff, players, facilities and ambition of the club it got me excited.
“Now I just can’t wait for pre-season to start and meet the players, staff and fans.”
Dawe has confirmed that Albion are on the verge of completing their first signings of the summer.
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