Conor Grimes gave the holders a dream start when palming home from close range in the fourth minute, while points by Shane O’Connor and Kevin O’Brien extended the hosts’ lead.
However, St Mary’s deservedly led 0-8 to 1-4 at the break thanks to scores from distance by Aidan Maguire, Ryan Johnston, Kevin McKernan, Conor Clarke and Jerome Johnston.
The visitors’ task was made more difficult as Richard O’Callaghan was dismissed following an altercation with O’Brien in the 40th minute.
Conor Madden netted for DIT in the 49th minute but St Mary’s replied in kind thanks to a goal by centre-back Daniel McBride.
However, late insurance scores by Madden, O’Brien and Bernard Deay were enough for DIT.
Scorers for DIT: C Madden 1-1, C Grimes 1-0, K O’Brien and P Acheson 0-2 each, S O’Connor, N Devereux, T O’Connor, K O’Connell, B Menton and B Deay 0-1 each.
Scorers for St Mary’s: D McBride 1-1, C Clarke 0-3, N Morgan (1 45, 1f) and R Johnston 0-2 each, C McMahon, J Johnston, K McKernan, A Maguire and E Bradley 0-1 each.
DIT: C Ferris (Dublin); G O’Hare (Louth), B Menton (Meath), M O’Hehir (Sligo); K O’Brien (Dublin), N Devereux (Dublin), K O’Connell (Monaghan); T O’Connor (Kildare), P Acheson (Tipperary); D O’Reilly (Dublin), C Reddin (Dublin), S O’Connor (Dublin); B Allen (Offaly), C Grimes (Louth), H Dawson (Dublin).
Subs: N Scully (Dublin) for Dawson (41), M Argue (Cavan) for Grimes (43), C Madden (Cavan) for Allen (47), B Deay (Kildare) for O’Reilly (55), R Sheridan for O’Hehir (61).
ST MARY’S: N Morgan (Tyrone); M Murray (Armagh), N Donnelly (Down), A Maguire (Antrim); C Clarke (Tyrone), D McBride (Tyrone), R Loughran (Tyrone); P Carragher (Armagh), B Hasson (Antrim); R Johnston (Down), K McKernan (Down), C McMahon (Tyrone); T Canavan (Tyrone), J Johnston (Down), R O’Callaghan (Fermanagh).
Subs: R Sexton (Down) for McMahon (36), E Bradley (Derry) for Murray (54), A McGarrity (Tyrone) for Hasson (59).
Referee: J Hickey (Carlow).