England began 122 behind after 20 wickets fell on the first day but took lunch with the scores level having lost only Rory Burns in the session.
Leach was an unlikely candidate to restore calm after their previous batting woes but he was unbeaten on 60 at the break – just six short of his highest first-class score – while Roy grew in confidence to reach 52no.
While the pair's partnership has steadied England's position in the match, and piled the pressure back on a tiring Irish attack, concerns over Burns will temper that satisfaction after he nicked Boyd Rankin for six.
His first home Test has yielded just 12 runs in 51 deliveries, lowering his overall average to 22.29 in the process, and he remains an open question at the head of the innings.
Roy settled to his task as the visiting seamers toiled away in punishing heat but the fact that his innings came after the shine had been taken off the new ball means his credentials as a prospective Ashes opener are far from settled.
He took a while to assert himself as day one hero Tim Murtagh and Mark Adair probed outside off stump but his first four scoring shots were boundaries. Two of those were thoroughly convincing drives, one slightly streaky and one edged through the vacant third slip area.
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.
Leach deserved at least as much credit for his steadfast knock, though. Numerous openers have been tried and discarded by the selectors in recent years but few have put together innings as calm as Leach, a man averaging under 11 in first-class cricket with two previous half-centuries in 108 attempts.
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.