McDonagh arrived at The Oval in January 2006 from Shamrock Rovers, and was one of former manager Roy Coyle's final signings before resigning.

Arguably his best performance for the Glens came in a 4–1 win over Belfast rivals Cliftonville at the Oval in December 2006, where he made surging runs from midfield and scored with a directed header, and many hoped he would have continued this great form into the annual Boxing Day derby clash with arch rivals Linfield, but McDonagh was handed a two-game suspension by the IFA.

[2] Infamous for his disciplinary record, with 11 bookings to his name this season, he has gained a reputation with officials up all over the country after his alleged two-foot stamp on Cliftonville's Ronan Scannell in December's televised CIS League Cup Final, a game which Glentoran went on to win 1-0 thanks to a 32nd minute Gary Hamilton goal.

In February 2008 he was loaned out to Armagh City for one season[3] Following his release by the Glens, McDonagh signed for Glenavon.

[7] He subsequently signed for Longford Town in June 2012, scoring on his debut against Mervue United, in a 5–4 win for his new club.