Conor Hagan’s touch of class proved decisive to seal full points for 10-man St Mary’s.
Hagan, the Saints’ player/manager, steered home a free-kick late in the first half with what proved the key moment in the derby battle with Broomhill.
A dismissal inside the opening half-hour period left St Mary’s on the backfoot in terms of numbers but both sets of players struggled to carve out scoring opportunities.
A strong wind disrupted the standard of football but Hagan made his mark with set-piece success on 40 minutes.
Sean Brady had an opportunity to find the net but, ultimately, a single goal secured victory.
“It was a test of character with 10 men,” said St Mary’s Gerry Fay. “It was great to get the win under those circumstances.”