A 66th-minute sin-binning for Dragons hooker Steve Jones turned the course of a 12-try thriller at Lansdowne Road.
Leinster (25) 56
Tries: O'Driscoll, O'Kelly (2), Norton (2), Contepomi, Ronan
Cons: O'Meara (6)
Pens: O'Meara (3)
Dragons (29) 39
Tries: Forster, Breeze, Warlow, Anthony, Sidoli
Cons: Montgomery (3), Warlow
Pens: Montgomery, Warlow
The Welsh side led 39-35 at that moment, but after the yellow card Leinster scored 21 unanswered points.
The Dragons got a bonus point for four tries in the opening 20 minutes as they chased maximum points in the hope of securing the Celtic League title.
But the Scarlets' win over Ulster meant the crown was out of their hands even without Leinster's fightback.
An incredible first half saw seven tries and 54 points as centre Percy Montgomery inspired an almost sevens-like approach to the game.
Dragons' number eight Michael Owen put back-row partner Jason Forster over after
five and a half minutes.
Ireland skipper Brian O'Driscoll, earning his 50th provincial cap, moved a lovely left-wing switch with Felipe Contepomi to cross in the corner for an immediate
Lock Malcolm O'Kelly then claimed the first of his brace of tries to edge Leinster ahead, before the Dragons struck again.
First Ben Breeze was put over in the left corner by lock Peter Sidoli on the
quarter hour and then Craig Warlow charged down Girvan Dempsey to slide gleefully
Prop Chris Anthony was next over the line, but the Lions regained their composure towards the break with two Brian O'Meara
penalties and his conversion of James Norton's 39th-minute try.
With Leinster number
eight Victor Costello yellow-carded, Sidoli was sent clean through for the opening try of the second half.
Norton picked up a bonus point for Leinster with the first of his brace on the hour mark.
Then with Jones off the field for an offence against O'Meara, the Welsh were torn apart.
Tries followed for O'Kelly, Norton, Contepomi and an injury-time clincher for flanker
Niall Ronan as Leinster ended their barren run and Ruddock left his post to take
over as Wales coach on the back of a defeat.
"We went for it, there was no doubt the intent was there," said Dragons coach Mike Ruddock.
"For me, the turning point was when Montgomery went off with a suspected broken hand in the second half, after that we couldn't relieve the pressure and Leinster were too strong.
"The only consolation for me is if Monty ends up missing South Africa's Test with Wales in the summer!"
Leinster: G Dempsey, J Norton, B O'Driscoll, S Horgan, B Burke, F Contepomi, B O'Meara, R Corrigan (capt), S Byrne, E Byrne, L Cullen, M O'Kelly, E Miller, N Ronan, V Costello.
Replacements: S Brophy, G Hickie, B Gissing, A McCullen, D Dillon, B O'Riordan, J McWeeney.
Dragons: S Tuipulotu, H Luscombe, P Montgomery, A Marinos (capt), B Breeze, C Warlow, G Baber, A Black, S Jones, C Anthony, I Gough, P Sidoli, J Ringer, J Forster, M Owen.
Replacements: K Crawford, R Snow, R Thomas, R Oakley, R Jones, S Winn, G Arasa.
Referee: Gregg Davies (Scotland)