Leach had been sent in to negate a solitary over at the end of day one, effectively sparing Rory Burns and Jason Roy the burden, but he took on the role of leading man before falling for 92 on the second afternoon at Lord's.
When he resumed in the morning session England were 122 adrift, but as he walked off to a standing ovation they were 60 ahead on 182 for three.
No Englishman has ever made a hundred after being asked to fulfil what is usually a brief, defensive role and Leach made a compelling case to become the first as he struck 16 boundaries as well as single-handedly beating the derisory 85 England managed collectively in their first innings.
A quick scan of the left-hander's first-class statistics show just how unlikely his performance was – coming to the crease with an average under 11, a top score of 66 and a grand total of five runs in his previous four attempts.
But he batted with resolve, sound technique and a healthy slice of luck that saw him dropped twice before finally fencing Tim Murtagh to second slip.
Remarkably, the Somerset man and frequent number 11, managed to steal most of the limelight in a 145-run stand with Roy. The Surrey batsman made a confident 72 to effectively nail down his Ashes spot, but Leach's feelgood efforts continually drew the gaze.
Leach made a considerably better fist of his assignment than Rory Burns, who scratched around for six and looked less settled and less sound than his temporary partner.
When Burns nicked Boyd Rankin to wicketkeeper Gary Wilson it was a flat end to a disappointing first home Test which yielded a dozen runs and plenty more reasons for concern.
Roy was also nervy early on but still found a way to keep things moving, with his first four scoring shots all boundaries. Only one was thoroughly convincing, two more were slightly streaky and the other sailed off the edge through the vacant third slip region.
There was one major statement of intent as he found his feet, lashing Andy McBrine for six on the charge, and he reached his fifty in just 47 balls with an authoritative cut off Stuart Thompson.
That shot levelled the scores at lunch, by which point Leach had chiselled out a steadfast 60no, built around solid judgement and a series of clean drives.
Leaving well and playing straight with the full face of the bat, at one stage he hit four boundaries in seven balls in a performance that put the top order's first day efforts to shame.
England moved ahead for the first time in the match in the opening over of the afternoon session, Roy whipping Rankin to midwicket, but Leach was about to start dominating proceedings.
He was badly dropped on 72, Rankin drawing the error and Wilson inexplicably allowing the ball to spill out of both gloves. Emboldened, he went on the attack.
There was an uppish cut over backward point, a lob that evaded mid-off and a punched drive that skimmed to the ropes. One misfired at cover later and he had moved past 85, the derisory total Ireland had bowled their hosts out for on the first morning.
Roy attempted to reassert himself on proceedings with a booming drive at Stuart Thompson but paid with his stumps to end a promising outing in grisly fashion.
The stars looked to be aligning when he fenced Murtagh to second slip and was grassed by Adair, but when the veteran seamer created a replica chance three balls later the catch was taken.