Gordon D'Arcy had his first outing in four weeks on Sunday but he could not stop the Neath-Swansea Ospreys moving 12 points clear at the top of the Celtic League, as they held off a late Leinster surge for a 16-12 win at Donnybrook.
A 66th-minute Jason Spice try and the boot of both Shaun Connor and his replacement Matthew Jones - the talented Wales Under-21 player- helped Lyn Jones' men to only their third win in nine trips across the Irish sea.
With substitute prop Emmett Byrne in the sin-bin for preventing a quick release, Leinster pilled on the pressure late on and got in for their second try through captain Shane Jennings after Christian Warner's first-half effort had left them 5-3 up at the break.
Centre D'Arcy - who injured a hamstring in Ireland's 28-17 RBS 6 Nations win over Italy last month - played an hour and sparked the Leinster backline into action late in the first half.
Andy Farrell admits the process of selecting England's 50-man training squad ahead of the World Cup was rigorous - but the backs and defence coach also revealed how tough it was to leave some stars out.