The 22-year-old travelled to Galway as 24th man but was drafted in at late notice.
Leinster only put the word out about an hour to kick-off at The Sportsground but there had been an inkling, for Jimmy O'Brien and his teammates, that there may be a late change to the starting XV.
Robbie Henshaw had been due to play his first Leinster game of the season, three weeks after featuring in Ireland's World Cup quarter final defeat to New Zealand, but a bout of sickness forced Leo Cullen and Stuart Lancaster to draft in O'Brien.
The Kildare native was called in for only his sixth start for the senior team and only his 12th appearance in total. He had Joe Tomane inside him and was facing off against a Connacht midfield of Peter Robb and Tom Farrell.
As it turned out, the Leinster front five would do most of the heavy lifting, blasting and scoring in the first half. Nonetheless, O'Brien impressed many of his senior colleagues with an impressive display, in attack and defence.
Ahead of the Guinness PRO14 fixture, Connacht had spoken of the game being a change for "revenge" after Leinster had scored a late try to beat them at The RDS, last season. Those words, broadcast by TG4, must have filtered back to the league champions as they were amped up from the very first minute.
By half-time, with Connacht's defence breached five times, Ronan Kelleher and Andrew Porter had scored two tries apiece and Ross Byrne, having scored a try himself, added all five conversions.
After the break, with Connacht knocking on the door and trying to fight their way back into the tie, O'Brien stepped up with a big hit against a bigger man, the 6-foot-3 Farrell:
https://twitter.com/SportTG4/status/1192915862518415360
It was one of the 10 successful tackles he made in the game while he made ground on each of his carries. Alongside him, Tomane had one of his best games in Leinster blue since he joined in the summer of 2018.
Elsewhere, Hugo Keenan was impressive in his 60 minutes at fullback while Lowe and Scott Fardy set a fast tempo and Will Connors registered 23 tackles (missing none) in his 72 minutes on the pitch. Several of the younger and fringe players are pressing decent claims to be involved in next Saturday's Champions Cup clash with Treviso next weekend.
Connacht restored a measure of pride in the second half, getting over for a try of their own and pressing for more until James Lowe intercepted a Dave Heffernan pass and scampered 70 metres to get the final score of the game.
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