Momentum swung in Derry’s favour when St Mary’s wing-back Michael Hynes had to be stretchered off with a serious knee injury.
Barely 90 seconds later, Kerrigan had his second goal in four days as he aided the Oak Leafers into a 1-7 to 0-7 interval lead.
The Claudy man finished with 1-7 as the strength of Derry’s bench, most notably with Fergal Doherty and Enda Lynn, saw them through.
DERRY: E McNicholl; G O’Kane (0-1), D McBride, D Brown; K Johnston, M Craig, J Kearney (0-1f); P McGuigan, P Cassidy; N Holly, N McFeely (0-1), N McNicholl; A O’Neill (0-1), C McGoldrick, A Kerrigan (1-7, 0-1f)
Subs: E Lynn (0-3) for N McNicholl (39), C McFaul (0-2) for McFeely (39), F Doherty for Cassidy (42), M Lynch (0-1) for McGuigan (42), A McAlynn for Brown (47), C Kielt for McBride (66)
ST MARY’S: D McAnenly; S Mullan, N Donnelly, M Murray; R Loughran, A Maguire, M Hynes; R O’Callaghan (0-1), P Carragher (0-1); K McKernan, T Canavan (0-6, 0-5f), R Johnston (0-1); J Duffy, R Lynch (0-1), B Hasson (0-2).
Subs: C McMahon for Hynes (20), A Forker for Lynch (39), R Sexton (0-1) for Carragher (39), J Grugan for Duffy (49), C McCann (0-1) for Donnelly (64)
Referee: I Molloy (Donegal).