An injury-time try from Eoin O'Malley helped Ireland score their first win of the IRB Under-19 World Championship at Ravenhill as they pipped Scotland 13-12.
The Irish laid siege to the Scottish line for the closing ten minutes and got their reward in the game's last play when centre O'Malley burrowed over.
Scotland led 12-6 with nine minutes remaining after centre Stephen McColl landed his fourth penalty of the evening.
The Scots led 9-3 at the break and defended tigerishly while hooker Adrian Duncan was in the sin-bin. But Ireland bounced back as Deasy, who kicked two penalties, converted their crucial late try.
Despite O'Malley's opening kick-off not going the required 10 metres, Ireland had the first points on the board in the third-minute. A high tackle by Neale Patrick allowed PBC Cork out-half Deasy float a left-sided kick through the posts from just outside the 22.
Playing into a strong wind, Ireland were always going to struggle for territory in the first half and only some extremely disciplined defending, with the pack outstanding in their ability to turnover the ball, kept Charlie McAleese's side in contention.
Deasy knocked a lengthier penalty attempt off the crossbar before McColl equalised with a tenth-minute kick that bounced over off an upright.
Irish skipper Paul Ryan was guilty of failing to release at a ruck and McColl booted Scotland into a 26th-minute lead.
After spending most of the opening half-hour in their own territory, Ireland launched a great attack through hooker Stephen Douglas, straight off a lineout, but they were met by a wall of blue.
Conor Cleary was harshly penalised for a ruck offence, four minutes from the interval, and McColl raised the touch judges' flags for a third time to send the Scots in at half-time with a 9-3 advantage.
Cleary was replaced by Jamie Smith at the break, with O'Malley moving to inside centre, and Ballymena starlet Smith went on to have a stand-out second half, which involved him putting in two superb try-saving tackles.
With the elements at their backs, Ireland pressed for the opening try and got within range when Deasy cut through a midfield gap. A series of phases later, flanker Rajan Reilly lunged for the try line but he lost the ball forward.
Deasy did punish Scottish skipper Frazer McKenzie for not rolling away when he kicked his second penalty, but Scotland managed to maintain their three-point lead despite losing Duncan to a yellow card.
Just before that offence, which saw Duncan binned for preventing a quick ruck release, McColl pushed a penalty wide of the posts and the tie looked to Ireland's for the taking.
Still, McAleese's side got a glimpse of just how potent the Scottish backs can be in the 55th-minute. A loose pass from Deasy was intercepted by his opposite number Ruairidh Jackson and the number ten took off from his own 22.
Showing a great turn of pace, substitute Smith tracked him all the way and forced him to offload to the supporting Lee Jones, but the winger was then brilliantly bundled into touch by the tigerish Smith whose efforts just about summed up Ireland's will to win.
Australian referee Julian Pritchard was then forced off with an injury, to be replaced by Welsh touch judge James Jones, who was the man in the middle for Ireland's 15-10 defeat to Australia last week.
Despite being down to 14 men, Scotland then had a spell of pressure which resulted in McColl sending over his fourth penalty in the 61st-minute.
Irish hopes faded as Paddy Brophy lacked support when he went on a solo run out of the 22 and he was let down by a poor pass from his fellow winger Niall Morris as the home side looked to have space out wide on the left.
But Ireland carved out a late penalty opportunity which Deasy kicked to the left corner. The lineout was won and despite repeated drives towards the whitewash, with Douglas, Reilly, Ryan and replacement Jason Harris-Wright right in the thick of it, Scotland kept holding out.
It was massive defensive effort from the Scots as they held the hosts up over their line on three occasions. However, the pressure eventually told when O'Malley was shunted over to the left of the posts in injury-time, sparking mass celebrations both on the field and off it.
Deasy knocked over the clinching conversion to move Ireland up to seventh in the pool standings. They face sixth-placed side South Africa on Thursday with both nations still in contention for a semi-final spot.
The Baby 'Boks lost 17-8 to France in this evening's earlier game at Ravenhill.
Scorers:
Ireland: Try: Eoin O'Malley; Con: Scott Deasy; Pens: Scott Deasy 2
Scotland: Pens: Stephen McColl 4
IRELAND: Adam Craig; Niall Morris, Eoin O'Malley, Conor Cleary, Paddy Brophy; Scott Deasy, Harry McAleese; Paul Karayiannis, Stephen Douglas, Matthew Nelson, Brian Cawley, Ian Nagle, Kieran Essex, Rajan Reilly, Paul Ryan (capt).
Replacements: Michael Sherry, Ian Leonard, Patrick McCabe, David Foley, Patrick Mallon, Kris Greene, Alan Gaughan, Jamie Smith, Murray Kinsella, Tommy Seymour, Jason Harris-Wright.
SCOTLAND: Murray Allan; Lee Jones, Chris Kinloch, Stephen McColl, Tom Bury; Ruairidh Jackson, Ross Samson; Gary Strain, Adrian Duncan, Joe Stafford, Neale Patrick, Josh Brown, Fraser McKenzie (capt), Kevin Bryce, Alexandre Wukovits.
Replacements: Ralph McInally, Angus Dixon, Lewis Niven, Craig Charters, Steven Burton, Mathew Heeks, James Murray, Ruairidh Bonner, Redha Guedroudj, Craig Simmonds, Harry Duthie.
Referee: Julian Pritchard (Australia)
