Teenage winger Jaze Kabia scored his first top-flight goal as Shelbourne did just enough to take the three points from their Dublin rivals at Tolka Park.
On a bittersweet night for the 19-year-old, however, he had to be helped from the field late on with a leg injury.
The victory marked Shelbourne’s first over St Pat’s in the league since 2006 when the Drumcondra side won the title prior to their financial implosion.
Shelbourne survived some early pressure to play their way into the game and punish negligent defending to take a deserved lead on 23 minutes for what would prove the only goal of the game.
Kabia was left completely unmarked to score with a downward header from Karl Sheppard’s corner with goalkeeper Brendan Clarke rooted to the spot.
In a tight, full-blooded derby, St Pat’s pinned their hosts back before the interval with Billy King bringing off a smart save at the foot of a post from Jack Brady in the final minute of the half.
St Pat’s dominated possession from the resumption as they chased the game.
Brady had to do well again on the hour mark to gather a Chris Forrester shot at the second attempt following a Jason McClelland corner.
St Pat’s hopes of getting back into it suffered a setback on 70 minutes when right-back Rory Feely, booked in the first half, received a harsh second yellow card and was sent off for what appeared an accidental trip on Aaron Dobbs.
SHELBOURNE: Bray; Friel, O. Brennan, O’Reilly, O’Hanlon; Kabia (Rooney, 81), Deegan, R. Brennan, Sheppard; Farrell (Poynton, 51), Dobbs.
ST PATRICK’S ATHLETIC: B. Clarke; Feely, Desmond, McNally, Bermingham (Walker, 81); D. Clarke, Markey, Forrester, King; McClelland (Doona, 61), Hale (Rennie, 61).
Referee: Sean Grant (Wexford).