The 28-year-old New Zealander joined from Hawkes Bay Magpies in 2013 and was named club captain for the 2014-15 season when he led Edinburgh to become the first Scottish club to reach a European final in the Challenge Cup.
Coman said: “It was a really tough decision to move on. Edinburgh feels like home now and I’ve had a great two-and-a-half years here, but moving on to a new challenge feels like the right decision at this time.
“The club has changed a lot since I joined, Solly [Alan Solomons] and the other coaches have done a great job.
“When I first arrived there were a lot of changes going on. When look around now, I see a club becoming sustainable with a lot of young guys who are shaping up to be great for Edinburgh Rugby.”
Coman has 48 Edinburgh appearances to his name and led the club to a second straight 1872 Cup success over Glasgow at the start of the year.
Edinburgh faced London Irish in the pool stage of this season’s Challenge Cup, losing away and winning at home as they were edged out of the quarter-finals by the Exiles.
Coman said: “Having spoken to (Irish head coach) Tom Coventry, who I know well from New Zealand, and seeing where he wants to take the club, I was happy to sign.
“Everything looks to be in place off the field to ensure the side are successful on it, and I look forward to being part of a squad working hard to achieve that.”
Edinburgh coach Alan Solomons said: “Mike will be sorely missed by all at Edinburgh Rugby.
“His contribution both on and off the field has been immense. He is an outstanding leader, a terrific bloke and a really good rugby player.
“Mike is one of the finest men I have met and it has been a pleasure and a privilege to coach and work with him. On behalf of all at Edinburgh rugby we wish him well in his new venture.”