Lions hopeful 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.
D'Arcy, who has been widely tipped to make the Lions summer tour to New Zealand, was unable to halt the Ospreys as they ran out 16-12 winners 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.