Ireland ended the penultimate day of their fixture against Australia A on 50-3 after being set 300 for victory.
After Australia A opted to declare overnight on 312-9, Paul Stirling starred for the hosts with 115 not out before they pulled out at tea on 186-4.
The Middlesex batsman dispatched pace bowler James Pattinson for three boundaries in the first over to set the tone for another dazzling hundred - his eighth for Ireland.
He reached his half-century from 44 deliveries and had hit 17 fours and a solitary six by the time he was stumped by Australia captain Brad Haddin off the bowling of Nathan Lyon.
Andrew White played his part in a third-wicket stand of 96 and he had reached 40 not out by the interval, in which time skipper O'Brien, who himself was unbeaten on 16, opted to declare.
Australia debutant Fawed Ahmed went wicketless but did improve after a nervous start, finishing with 0-40 from his 11 overs. Lyon, meanwhile, conceded just 34 runs in his 16 overs of work.
Looking to set up a final-innings run chase, the tourists duly came out and amassed 174 without losing a wicket in 20.2 overs, mainly down to Nic Maddinson.
The New South Wales opener Nic Maddinson smashed 113 from just 63 balls, including 13 fours and six sixes. Alex Doolan was happy to play second fiddle, though he was hardly a slouch in reaching 56 not out.
With Haddin setting the hosts 300 in just over a day, Australia A quickly saw off Stirling second time around when Pattinson had him caught behind for six.
Moises Henriques then dismissed John Anderson (eight) and Alex Cusack (15), leaving James Shannon (11 not out) and White (2 no) to see Ireland through to the close without any further damage done.