The younger brother of Scotland scrum-half Mike Blair converted his own 72nd-minute try to see the Scots retake the lead and hold on for a famous win over Ireland.
Having beaten Ireland only once at the under-21 grade - that being last year's 16-8 success at Falkirk Stadium - the victory at Dubarry Park was made all the sweeter.
Iain Paxton's Scots - with four Warriors in the squad - were on the back foot for much of the opening quarter as the hosts, beaten by both France and Wales in their last two outings, sought retribution.
They looked ideally placed to end their drought when recalled fly-half Jonathan Sexton landed three early penalties to put them into a 9-0 lead.
A yellow card for Glasgow apprentice Iain Kennedy after 23 minutes aided Ireland's cause, but crucially, Ireland failed to garner a try with the visitors down to 14 men.
A brace of penalties from Blair got the Scots off the mark and with Irish prop Calum Black in the sin bin for slowing up ruck ball, Paxton's youngsters struck for a sucker punch try, five minutes into injury time.
Leicester Tigers prop David Young, who was incidentally born in Belfast, plunged over after a concerted series of drives from the Scots. Blair missed the conversion, but his side still had a 11-9 lead to take into the second half.
Blair added his third penalty before a neat back line move on 47 minutes saw UCD centre Fergus McFadden slalom through for Ireland's opening try. Sexton converted for 16-14.
The nip and tuck nature of what was an entertaining game continued when Blair booted his fourth penalty on 58 minutes.
However, seconds later, a lightning Irish attack saw full-back Fionn Carr blaze through for a try in the corner, and what looked like the telling blow.
Sexton missed the conversion, and although on top for much on the remaining quarter, Ireland still looked shaky at 21-17 in front.
Blair then dramatically won it for his side when he dummied and cut through near the posts for the vital try that saw Ireland slip to their third straight defeat.
The news wasn't so good for the Scottish Club International team as they went down 30-13 to their Irish counterparts, but Scotland's women grabbed their first win in this year's Six Nations with a 9-0 win over Ireland .