But Leinster responded to the challenge and their four-try demolition of Ulster, allied to Connacht's 25-24 victory over Munster in Ballincollig, ensured that the McGee Cup is staying in the eastern province.
O'Brien's charges led Ulster 10-7 at half-time with Johnny Butterworth touching down for a try and Wicklow clubman Barry Lynn kicking a penalty and conversion to Ulster's converted try from Andrew Semple.
Lynn, though, broke his hand just before half-time and had to be replaced by the 19-year-old Harry Kennedy.
Leinster started the second half well with Damien Broughall touching down for a try. Brian Collins added the extras.
Leinster suffered a blow shortly afterwards. Former Ireland underage international Collins had to retire due to an ankle injury. He was replaced by Dundalk youngster Gearoid McDonald whose first job was to knock over a penalty to leave Leinster leading 20-7.
Ulster, who lost 21-14 to Munster last time out, edged closer to Leinster with a second converted try, scored by replacement Andy Kennedy from Cooke.
At 20-14 and with 20 minutes left, the game was in the melting pot but as has been their forte in this series, Leinster finished very strongly with replacement scrum half Peter Sutton notching their third try.
With minutes left on the clock and after great work from David Murphy, the visitors added the coup de grace with McDonald touching down for the fourth and final try.
It was Leinster's young replacements, Peter Sutton, Harry Kennedy and McDonald, who turned the game in the visitors' favour with their energy, enthusiasm and energy.
The result saw Leinster stretch their unbeaten run at this level to eleven matches. Afterwards, Kilkenny clubman John Phelan was presented with the trophy.
Leinster awarded 20 new caps over the three interprovincial games and their policy of youth paid dividends - the average age of the Leinster back-line was 21.
O'Brien and his management team used 30 representatives from 16 different clubs during the 2010 campaign, which is a significant testament to the players on how well they gelled together in such a short space of time.
The province's five selectors - David Coughlan, Mick Clavin, Trevor Wardrop, Bill Douglas and Ger McDonald - also had some vital insight and input into this fresh Leinster set-up.
Meanwhile, Munster missed out on a great chance to win the title as Connacht snatched a dramatic 25-24 win at Ballincollig RFC.
Two tries in the final five minutes from Martin Lee and Kevin Corcoran saw Connacht cause an upset at Tanner Park as they defeated Munster by the narrowest of margins.
Munster can have no complaints about this result as they were outplayed for long periods, particularly at scrum time, and the visitors used the conditions to their advantage.
Both sides adopted an expansive style and Connacht opted to run kickable penalty chances as they used the strong wind to keep Munster pinned back in their own half.
The westerners opened the scoring on twenty seven minutes when the excellent Corcoran outpaced the Munster defence to touch down in the corner.
This score woke Munster up as they raised their game and were level just four minutes later when, after a series of drives at the Connacht line, Glen Deacon touched down.
Connacht were back in front shortly afterwards, when a great break by out-half Paddy Pearson set up Jordan Farrington for a wel-taken try which Pearson converted.
A penalty by Pearson on the stroke of half time left it 15-5 in favour of the westerners who were the only side out of contention for the Championship on the final weekend.
Munster came out in the second half a changed side and within a minute they had the deficit down to three points, with David Joyce getting in for a try which Darragh Meade converted.
By the fifth minute of the second half, a rejuvenated Munster were in front when out-half Meade got in for a try.
Midway through the second half a charging run by replacement Diarmuid O'Connor set up a try for winger Barry Gibbons which Meade converted.
With the bonus point in the bag and news that Leinster were just edging their tie away to Ulster, the title looked like it was heading south, but Munster failed to close the game out.
Instead it was Connacht who threw caution to the wind. Last year's runners-up ran the ball at every opportunity and were rewarded for their endeavour.
Hooker Lee touched down and then a minute from time Corcoran again showed his pace to touch down and give Connacht a single-point advantage. A shellshocked Munster could not hit back and their hopes of Championship glory were gone.
2009/10 IRFU JUNIOR INTERPROVINCIAL CHAMPIONSHIP:
Saturday, April 24 -
Leinster 15 Munster 15, Seapoint RFC
Ulster 34 Connacht 23, Virginia RFC
Saturday, May 1 -
Connacht 17 Leinster 23, Creggs RFC
Munster 21 Ulster 13, Abbeyfeale RFC
Saturday, May 8 -
Ulster 14 Leinster 32, Coleraine RFC
Munster 24 Connacht 25, Ballincollig RFC
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