UCD manager Collie O'Neill had described his players as being like "kids on the night before Christmas" and he must have felt as if all his Christmases had come at once when a defensive error allowed his side claim a famous win in Europe.
Their goal had an element of good fortune to it but UCD's tireless work rate more than merited their first-leg victory.
F91 Dudelange were the better side for large parts but they struggled to break down the Students' well-organised unit.
UCD barely threatened in the first half and in fact scored with their first shot on goal when 19-year-old Ryan Swan - son of former League of Ireland striker Derek - stunned the side from Luxembourg.
Robbie Benson nicked the ball off Joel De Almeida Pedro on the edge of his own area before playing Swan (right) through on goal and the striker finished well.
The Students had Niall Corbet to thank for keeping them in the game after the 'keeper pulled off a string of fine saves.
Sanel Ibrahimovic rattled the post with a stinging drive midway through the opening half while Sofian Benzouien went close on a couple of occasions.
The pacey Luxembourg outfit looked threatening on the break throughout but UCD rode their luck and completely against the run of play, snatched the goal in first-half stoppage time.
Ibrahimovic should have equalised shortly after the restart but he failed to connect with a cross.
The home side continued to sit back and soak up the pressure and they very nearly added to their tally when Chris Mulhall's 30-yard shot was tipped over the bar.
Dudelange substitute David Turpel failed to keep his composure when put clean through on goal as frustration levels began to grow in his side.
Benson almost capped a memorable evening but his dipping volley late on hit the crossbar but, in truth, UCD will be more than happy to travel to Luxembourg next Thursday with a precious 1-0 lead.