Played in Steelstown, Co. Derry in glorious conditions St. Eunan's dominated from the start and led at half time by 7 pts. to 2.
They continued to dominate with St. Mary's scoring a goal in the last minute.
Rory Carr, Michael Miller and David Tyrell were impressive for the Letterkenny college.
St. Eunan's have now qualified for the play offs where it looks like they will meet either St.Patrick's Armagh or St. Patrick's , Dungannon.
In their final league match they play St Mc Cartan's , Monaghan on Tuesday, venue to be decided.