Mooney, who played six games with the Magpies after making an impressive debut against Fremantle in round 14, 2012 posted a tweet on Wednesday night that he had decided to return to Ireland.
The 21-year-old joined the Magpies from Rostrevor three years ago and earned a reputation as a courageous speedster although he failed to play a game in 2014.
Collingwood's disappearing act
Reports from Ireland also suggested Clarke was set to return to his home country, completing a 73-game career with the Magpies that began in 2007. His departure was subsequently confirmed by Collingwood on Thursday morning.
Clarke, 26, began his career in outstanding fashion in 2007 and was an exceptional performer in that first season, playing a key role in the Magpies' defence as they just failed to win a preliminary final against Geelong.
He played 22 games in 2008 before struggling to secure a spot in the team after the midway point of 2009.
He returned to Ireland but came back to the Magpies two years later when Nathan Buckley began as senior coach in 2012.
Clarke established himself as a tagger but was not as effective as he had been at the start of his career, struggling to stay with the pace of the game in his comeback.
He was a popular figure at Collingwood and one of the more successful Irish imports, renowned at the Magpies for his endurance and running power and he played in five finals.
The Pies announced the list changes on Thursday, making a total of seven players who will depart the club, with Nick Maxwell, Luke Ball, Ben Hudson and Quinten Lynch retiring this season.