Kirby (19), who was making just his second senior start, netted four times between the 23rd and 27th minutes.
In a young looking Mayo team, he was ably supported by debutants Evan Regan and Conor O’Shea, a brother of Seamus and Aidan, who both scored.
The winners led 4-6 to 0-4 at half-time but GMIT battled bravely to the end with top scorer, Jonathan Ryan, Diarmuid Cronin, Eoin Ward and Aidan Walsh all grabbing consolation scores.
Scorers for Mayo: D Kirby 4-1, C Mortimer 0-6 (4fs), P Gardiner, C O’Shea, D Coen, E Regan 0-1 each.
Scorers for GMIT: J Ryan 0-5 (4fs), A Walsh 0-2 (1f), C Sweeney, D Cronin, P Sweeney, E Ward 0-1 each.
MAYO: D Clarke; E O’Reilly, G Cafferkey, M Walsh; P Gardiner, K Higgins, L Keegan; B Moran, P Harte; C O’Shea, D Coen, K McLoughlin; E Regan, D Kirby, C Mortimer.
Subs: J Gibbons for Harte, A Feeney for O’Reilly, E Varley for Regan, S O’Shea for C O’Shea.
GMIT: B Walsh; M Kelly, A Tierney, D Lee; C Sweeney, J Duane, J McLaughlin; A Walsh, S Boyle; D Durkan, J Ryan, N Prenty; D Cronin, P Sweeney, N Farragher.
Subs: E Ward for Cronin, B Sheridan for Kelly, D McIntyre for Farragher, D Clabby for Boyle, D Quinn for P Sweeney.
Referee: TJ Keaveney (Sligo).