St Mary's 35
An injury time try by Victor Costello, converted by Mark McHugh broke DLSP hearts in an eight-try thriller at Templeville Road.
DLSP trailed 18-3 after 22 minutes but scored four tries to lead 18-30 with 20 minutes remaining.
St Mary', however, kept their play-off hopes alive with tries for Conor McPhillips and Costello's late winner.
DLSP picked up two bonus points and could yet avoid the drop if they can beat Galwegians in a fortnight's time.
Scores in brief
At Templeville Road:
St Mary¹s 35: P McKenna, G Gannon, C McPhillips, V Costello tries; M McHugh
3 pens, 3 cons
DLSP 33: B Hogan 2 tries, D Masters, S McCarthy try each; S Broughton 3
pens, 2 cons.