St. Mary’s College overturned a half-time deficit with four second half tries to condemn former champions Garryowen to relegation for the first time in their history.
Garryowen had the unique distinction of being the only club to have played at the top level of the All-Ireland League every year since the competition began.
But that remarkable run was ended by a 40-26 defeat on Saturday as St. Mary's swung an enthralling 10-try see-saw of an encounter in their favour with 21 unanswered points after the interval.
Tries from JP Cooney, Mark Rowley and Steve McMahon with two Andrew O'Byrne conversions gave Garryowen a 19-12 lead in an end-to-end first half.
St. Mary's scores in the opening period came courtesy of tries from captain Kevin Sheahan and Marcus O'Driscoll plus a Cathal Marsh conversion.
But converted Dylan Fawsitt, O'Driscoll and Ryan O'Loughlin tries put the hosts firmly in the driving seat at 33-19.
Replacement Jamie Gavin converted captain Neil Cronin's try to give the visitors a try-scoring bonus point. However, they were denied a losing bonus point when St. Mary's replacement Gareth Austen crossed in the dying minutes.
Three-time league winners Garryowen had faced the prospect of relegation all season having lost their first 10 games in Division 1A.
The arrival of ex-Ireland head coach Eddie O'Sullivan sparked a mini revival with three wins, but recent defeats to Clontarf and Dolphin doomed them to Division 1B rugby for next season.
St. Mary's, on the other hand, had all but secured their Division 1A status following a bonus point victory at Lansdowne last Saturday.
Tries from Leinster pair Darren Hudson (2) and Darragh Fanning helped secure a 24-15 success for the Dubliners in the previous meeting with Garryowen at Dooradoyle back in January.
This time around, the Limerick club needed a sizeable bonus point win and for UCD to keep Ballynahinch pointless to have any chance of reaching the relegation play-off.
And the size of their task became even more daunting when number 8 Sheahan barged over on eight minutes with Leinster 'A' out-half Marsh adding the extras to put the hosts 7-0 up.
To their credit the Light Blues came storming back and tied the game in the 13th minute when prop Cooney stretched over the line and O'Byrne successfully landed his conversion.
Full-back Eamonn Connolly attempted a drop goal and flanker Anthony Kavanagh was held up as the Munster outfit continued to press for more scores.
And on 24 minutes Rowley grounded the ball off a forwards' drive to give rock bottom Garryowen a five-point lead.
Despite playing with the sun in their faces, St. Mary's hit back straight away when winger O'Driscoll crossed in the corner to level the match on the half hour.
But just four minutes later, centre McMahon slithered his way through the St. Mary's defence to cross the whitewash. O'Byrne converted to give the division's bottom side a seven-point advantage at the interval.
However, within five minutes of the restart, home hooker Fawsitt finished off an overlap and out-half Marsh landed a difficult kick to draw the teams level once again.
The hosts then regained the lead in the 54th minute when O'Driscoll touched down for his second try. Marsh converted to put the St. Mary's 26-19 in front.
They stretched the margin to 14 points on 66 minutes when centre O'Loughlin went over and Marsh brilliantly converted from the touchline.
Garryowen though refused to throw in the towel and with 10 minutes left scrum half Cronin broke free to score with replacement Gavin reducing the arrears to a converted try.
The gutsy Limerick outfit poured forward in search of a fifth try, but St. Mary's broke and were almost in when replacement Robbie Glynn over-kicked his punt out of play.
Marsh missed his penalty attempt with a minute to go, but in injury-time he made amends by converting Austen's try which sealed Garryowen's fate.
Referee: David Wilkinson (IRFU)