Former Student Robbie Martin put Longford Town into their fourth FAI Cup final in seven years with a cracking winner at Belfield.
Alan Mathews' side, bottom of the Premier Division table all season, now meet Cork City in this year's decider at the RDS on 2 December.
The result was harsh on UCD, bidding to make the final for the first time in 23 years, as they had the better of the possession and chances.
But Longford defended well with man-of-the-match Damien Brennan a colossus at the back and keeper Shay Kelly making crucial saves.
Longford had the first sniff of goal after just two minutes. Martin set-up Dave Mooney on the edge of the area, but the league's 16-goal topscorer miscued well wide.
In an open game, College responded five minutes later. Patrick McWalter split the Longford defence to put Francis Moran in on goal. Kelly stood up well to make the stop with his feet.
Ronan Finn then began and finished the best bit of football of the first half on 18 minutes.
Sweeping the ball out wide right, midfielder Finn then raced into the box to get on the end of Alan Mahon's low cross and bring another smart save at his near post from Kelly.
But UCD had some defending to do within two minutes when Dessie Baker and Mooney combined inside their box with the latter having his shot blocked by Conor Kenna.
Darren Quigley in the UCD goal then saw action for the first time on the half hour, getting out sharply to save with a foot from Mooney after Baker had put him through.
UCD remained the better and Kelly had to show his agility to touch over his bar to concede a fourth corner of the match on 36 minutes from Conor Sammon's looping header.
But the game got the goal it required to really fire it into life and it came at the other end on 39 minutes.
Jamie Duffy, ineffective up till then, did well to cut in form the right and set-up Martin whose low left-foot shot crept in at Quigley's right-hand post.
College might have been level within two minutes in what was now a real cup-tie.
Moran did well to put McWalter in down the left, but his chip was cleared off the line by Town skipper Brennan, who had a superb match.
UCD threw everything at Longford on the resumption with Kelly again stretching low down to push out Alan McNally's long range drive.
Longford were dangerous on the break and Quigley saved with his feet from Baker's low shot after Mooney's pass put his strike-partner away.
Mooney almost doubled Longford's lead on 64 minutes, but his audacious back-flick from Pat Sullivan's low cross was inches wide.
Quigley then deprived Mooney in stoppage time-with a stunning one-handed save.
UCD: Quigley; Mahon, McNally, Kenna, Bermingham; C. Byrne (P. Byrne, 84), Finn, McDonnell, McWalter; Moran (Forsyth, 84), Sammon.
Longford Town: Kelly; Sullivan, Brennan, Prunty, Rutherford; Duffy (Reilly, 85), Deegan, Doyle, Martin; Baker, Mooney.
Referee: Anthony Buttimer (Cork).