O'Brien's late try secures bonus point for clinical Clontarf
However, the impressive Jonny Hepworth sent Niall O'Brien through for a try deep in injury time to send the travelling supporters home happy.
St Mary's young side started the game well, with veteran John McWeeney and Willie Duggan prominent early on. Six minutes in, out-half John Sexton threaded a clever kick through the Clontarf cover and James Norton won the race for the touchdown.