Despite dominating from the kick-off it took UCD 13 minutes to register a chance, with Paul O’Conor shooting wide from Robbie Benson’s through ball.
Gary Burke and Rusk wasted further openings before O’Conor was played through by the latter to dink the ball over Tadgh Murphy for the opening goal on 27 minutes.
Burke rattled the crossbar with a 30-yard drive minutes later before Phoenix levelled the first time they got near the UCD box on 33 minutes.
Samir Belhout barged John Brophy to the ground, with Keith Dawson scoring emphatically from the penalty spot, sending Mark McGinley the wrong way.
One-way traffic resumed in the second half, however; Ledwith striking the inside of a post from distance on 59 minutes before the goals arrived. Right-back Ledwith curled home a stunning free-kick to restore the students’ lead on 75 minutes. Leading scorer Rusk added the third goal on 81 minutes, side-footing home sub Dean Clarke’s cross.
UCD: McGinley; Ledwith, D. O’Connor (Douglas 83), Leahy, Nangle; Belhout (Clarke 65), P O’Conor, Corry, Benson, Burke (Langtry 65); Rusk.
PHOENIX: Murphy; Sheridan, Philips, Kearns, Kelly; Gleeson, Cooney, Wade, Brophy (Kennedy 75); Giffney (Swan 73); Dawson (Ianolli 82).
Referee: Richie Winter (Wicklow).