Ospreys pipped Leinster at the death to claim a dramatic 31-30 win in Sunday's RaboDirect PRO12 final at the RDS.
In his last game for Ospreys, former Wales wing Shane Williams scored the match-winning try in the 78th minute - duly converted by Dan Biggar - to help claim the spoils for the Welsh region in Dublin.
Both sides scored three tries, but the home side paid the price for a missed early penalty by Jonathan Sexton - a weak kick which proved the difference as Biggar's sole miss was a conversion, meaning he kicked 16 points to the Leinster fly-half's 15.
Leinster mostly paid the price for indiscipline though - Heinke van der Merwe and Nathan White both spending time in the bin - conceding two of three tries at points when they were a man short.
Ospreys have now won the 'Celtic' League twice in three seasons, having also beaten Leinster in the 2010 final.
The Welsh region went into the game on a six match winning streak, but they found themselves 17-9 down at half-time with Sean Cronin and Isa Nacewa going over for the hosts. Sexton also added a penalty while Biggar responded with three of his own for the visitors.
Despite two more three-pointers from Sexton in the second period, touchdowns from Ashley Beck and Williams saw the gap reduced to two points before Nacewa's second score seemingly sealed the victory.
But, after Biggar had kicked a fourth penalty, Williams went over for a converted try to deny Leinster a European and league double.
It was the Swansea-based outfit who opened the scoring thanks to a three-pointer from their fly-half. However, Sexton converted a penalty opportunity to level matters.
Biggar kicked a second from the tee, but it was the Irish province that looked the most threatening with ball in hand.
Having failed to secure the double last season, losing to arch-rivals Munster in the play-off final, Leinster seemed determined not to replicate that lacklustre performance. And Cronin went over for the first try of the match after a superb off-load from Brian O'Driscoll, Sexton converting, to forge them into the lead.
The visiting fly-half reduced the arrears with another successful penalty before Nacewa pounced on some slack Ospreys' defence to touch down straight from the restart.
Van der Merwe, in for the injured Cian Healy, was then sin-binned after repeated infringements in the scrum, but the home side held on until the break.
The Ospreys took advantage of the extra man two minutes into the second half when Beck crossed the whitewash.
However, their good work was undone when ill-discipline allowed Sexton to kick successive penalties and extend Leinster's advantage to 23-16.
Once again, the visitors hit back through a Williams try in his final game for the region, but Nacewa touched down to relieve the pressure.
Nathan White became the second player to be yellow carded for the Irish province and Biggar kicked the resultant penalty before Williams provided the perfect send-off.
Tries: Cronin, Nacewa 2
Cons: Sexton 3
Pen: Sexton 3
Tries: Beck, Williams 2
Pens: Biggar 4
Leinster: 15 Rob Kearney, 14 Fergus McFadden, 13 Brian O'Driscoll, 12 Gordon D'Arcy, 11 Isa Nacewa, 10 Jonathan Sexton, 9 Eoin Reddan, 8 Jamie Heaslip, 7 Shane Jennings, 6 Kevin McLaughlin, 5 Devin Toner, 4 Leo Cullen (c), 3 Mike Ross, 2 Sean Cronin, 1 Heinke van der Merwe.
Replacements: 16 Richardt Strauss, 17 Cian Healy/Jack McGrath, 18 Nathan White, 19 Brad Thorn, 20 Dominic Ryan, 21 John Cooney, 22 Ian Madigan, 23 David Kearney.
Ospreys: 15 Richard Fussell, 14 Hanno Dirksen, 13 Andrew Bishop, 12 Ashley Beck, 11 Shane Williams, 10 Dan Biggar, 9 Rhys Webb, 8 Joe Bearman, 7 Justin Tipuric, 6 Ryan Jones, 5 Ian Evans, 4 Alun Wyn Jones (capt), 3 Adam Jones, 2 Richard Hibbard, 1 Paul James.
Replacements: 16 Scott Baldwin, 17 Ryan Bevington, 18 Aaron Jarvis, 19 James King, 20 Tom Smith, 21 Kahn Fotuali'i, 22 Matthew Morgan, 23 Tom Isaacs.
Referee: Romain Poite (France)