The 32-year-old is coming to the end of his contract having made 67 appearances for the club since arriving from Aberdeen.
The Irishman wrote on his Instagram account: "From the lows of breaking a leg to the highs of winning a treble, the last three years have been an enjoyable journey, in which I've worked under some terrific staff and shared a dressing room with some unbelievable guys!
Skipper Scott Brown said: "Going to miss you my man. What a man and what a player. Hard going in the dressing room to replace x."
Former Hoops left-back Kieran Tierney added: "What a guy mate. Memories nobody can ever take away."
Mikael Lustig said: "Honestly the best teammate you can have! Top player and decent stories. Was a pleasure to sit next to you during our years together. All the best in the future my friend."