The game at Rodney Parade proved hard-fought
Dragons: (15) 22
Tries: S Jones, Black (2)
Cons: Sweeney, A Thomas
Leinster: (10) 23
Indiscipline from the Dragons allowed Magners League leaders Leinster to take a hard-fought victory at Rodney Parade.
Tries: Blaney, Whitaker
Cons: Contepomi (2)
Pens: Contepomi (3)
Tries for Steve Jones and Adam Black put the home side in front, with Brian Blaney keeping the Irish in touch.
In a fiery encounter, Blaney and Paul Emerick were both sin-binned before Dragons duo Ian Gough and Ceri Sweeney both saw yellow 10 minutes from time.
A Felipe Contepomi penalty and Chris Whitaker try sealed the win, with Black's second try salvaging a bonus.
The result ended a 10-game winning streak in Newport by the Dragons, who were hoping for a boost ahead of next Saturday's European Challenge Cup semi-final at Clermont Auvergne.
But it helped mark a memorable day for Leinster prop Reggie Corrigan, who became the province's most-capped player with 135 appearances.
The game was within our control but we lost control at the crucial time
Dragons coach Paul Turner
After Contepomi missed an early penalty for the visitors, Sweeney landed an 11th-minute kick to open the scoring.
The Dragons then saw tight-head prop Lee Harrison limp off the pitch, prompting an earlier-than-expected introduction from the bench for Rhys Thomas, back after a mystery chest problem that had kept him out since October.
The Wales prop had a hand in the first try, taking the ball on from a line-out in the 22 before a delightful Michael Owen pass sent Jones in at the corner.
Gordon D'Arcy was predictably the spark for a Leinster comeback, a midfield break prompting a sustained attack from the league leaders.
A penalty came for a deliberate knock-on, but the visitors chose to run it and were rewarded as hooker Blaney barged over under the posts with a bullocking short-range run.
The game was getting feisty, with Gough warned for an attempted shoulder charge, but it was try-scorer Blaney who was sent to the sin bin when he raised his fist for a semi-punch in a maul.
The Dragons capitalised immediately with a strong forward drive, Gareth Cooper picking up at the back of the ruck and delivering a flat ball that Black took at pace to crash over under the posts.
Leinster's worries increased when Guy Easterby limped off with a worrying leg injury after falling awkwardly at a ruck.
But with half-time looming a strong attack by the visitors prompted Emerick to come in on the wrong side of a ruck, the centre sin-binned as Contepomi's penalty made it 15-10 with the last kick of the half.
The Dragons went close to stretching their lead straight after the break, a lovely chip over the defence by Sweeney falling to Gareth Wyatt, who failed to gather with the try line beckoning.
Bad-tempered off-the-ball clashes continued as the game became scrappy, a 58th-minute Contepomi penalty the only score of the third period.
The Dragons held out under Leinster pressure, but with 10 minutes to go Gough was sin-binned for a stamp at a ruck.
From the next phase of play a stiff-arm tackle from Sweeney saw the outside-half yellow carded as a mass fracas broke out.
Contepomi landed the resultant penalty to put his side ahead for the first time and in a winning position against 13 men.
It looked to be all over when Contepomi split the stretched defence to send Whitaker in under the posts.
But the home side hit straight back, Aled Brew setting off on a mazy run before sending man-of-the-match Black in for his second try to give the Dragons some consolation.
Dragons coach Paul Turner:
"We're really disappointed with the manner of that defeat because it was within our control.
"We lost control at the crucial time, first by giving away possession, then with our indiscipline.
"I'd like to look at some of the referee's decisions, but if you make those mistakes against a quality side like Leinster you will get it off them.
"It's a disappointed changing room, but we've come a long way this season."
Dragons: M. Thomas, Wyatt, Emerick, N. Brew, A. Brew, Sweeney, Cooper, Black, Jones, Harrison, Gough, Charteris, Charvis, Ringer, Owen, Thomas.
Replacements: A. Thomas for M. Thomas (57), Daly for Jones (63), Thomas for Harrison (12), Sidoli for Charteris (73), Bearman for Thomas (77). Not Used: Fussell, Walker.
Leinster: Dempsey, Horgan, Lewis, D'Arcy, Hickie, Contepomi, Easterby, Corrigan, Blaney, Green, Hogan, O'Kelly, Keogh, Gleeson, Heaslip.
Replacements: Whitaker for Easterby (40), Wright for Corrigan (36), Finegan for Hogan (67). Not Used: Vermaas, McLaughlin, Fitzgerald, Kearney.
Referee: Malcolm Changleng (Scotland).