That win, coupled with another big upset by the Irish against Pakistan in Kingston, Jamaica at the previous World Cup - where O'Brien was there at the finish alongside Trent Johnston - kickstarted the push for Ireland to get Test status.
That it hasn't happened is nearly single-handedly thanks to O'Brien, who first became the first man to register a 50 for Ireland - with pleasing symmetry it took him 100 balls, exactly twice as many as his extraordinary 2011 century. "If I continue on tomorrow for another hour and a half and that 118 changes to 170-odd, this could top it".
"It's a very proud and emotional moment", he said.
It was just after 6.15pm that Kevin O'Brien pushed a ball from Mohammad Amir through the covers for runs that made him a national cricketing hero, for the second time in his career.
Ireland had been fighting to prevent a heavy defeat in their maiden Test when, still 85 behind, O'Brien came out to bat at six, but his 118 not out, coupled with Stuart Thompson's useful 53, has raised hopes of an unlikely draw, or even an improbable victory. "Hopefully now we've put ourselves in a good position to try and go ahead and win it".
Ireland were also helped by the fact that Amir, who earlier on Monday took his 100th Test wicket, was off the field for much of the day's final session as he struggled with the recurrence of a knee injury that had flared up again on Sunday.
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And only three teams - one for every century in which Test cricket has been played - have won after following-on, with South Africa in 1894, England in 1981 and India at Kolkata in 2001, all beating Australia having been sent back into bat.
O'Brien and Thompson then began to drain the urgency from the fielding side, or maybe the visitors just fancied enjoying the local hospitality for another night, because taking the second new ball with only one or two slips in place and very little aggression all but ensured a fifth day - and what a thriller it could be.
Kevin O'Brien is hoping his latest worldwide ton will lead to an Ireland victory - just as his century against England did at the World Cup seven years ago.
Test cricket is a amusing game and Ireland will start to think of forcing the issue as they are 139 runs ahead with three wickets in hand.
And, though a ripper from Shadab Khan removed Thompson, O'Brien proved immovable, his belligerent strokes keeping the scoreboard ticking before he wrote his name in the history books by sneaking two through point to bring up his hundred.