Both sides had to wait almost six months to play the delayed final that should have taken place last season and it was a tightly contested game at Leixlip United last week until Alexandra pulled away after the break.
With the sides deadlocked at 1-1 at half-time, Hannah McDermott seized the limelight by inspiring the South Dubliners to victory with a second-half brace.
Beaten in the Leinster Junior and Senior Cup finals already this year, Alexandra began this game with purpose and were 1-0 up within five minutes of the kick-off.
Winger Aisling Nugent immediately proved a threat on the left and despite McDermott failing to finish her cross to the back post, Sophie Gleeson's resultant corner found Aideen Cowhey in the box and she finished from close range.
Despite Alexandra's early threat, St Mary's levelled the scores clinically with their first chance of the game 10 minutes later as a loose ball in midfield saw Shannon Doherty escape clear on goal and she rounded the goalkeeper before stroking the ball home.
Within 60 seconds the lead should have swung back to Alexandra as Nugent's skill on the left took her by two defenders and forced an acrobatic save from goalkeeper Aine Creah, only for the ball to strike a defender's arm.
Caroline Healy took the penalty but Creah again stood tall to make the save. Minutes later she received a huge slice of luck as a Katie Walsh shot from the edge of the box beat her all ends up but cannoned back off the underside of the crossbar.
Despite their dominance, Alexandra still needed their own stopper Ceithleann Ní Chonluin to deny Hayley Nolan on the stroke of half-time.
Despite McDermott finally separating the sides 10 minutes into the second half with a low shot, Mary's drew level for the second time within two minutes as Doherty's cross from the right hit the crossbar and Marie Clarke tapped in.
However, Alexandra were not to be denied and capped an incredible four-minute goal feast with their third within 60 seconds as Nugent finished into the corner after McDermott flicked on Anna Roopnarinesingh's cross from the right.
McDermott than sealed the victory in style five minutes later when she broke free on goal and finished coolly in the corner.
ALEXANDRA COLLEGE: Ceithleann Ní Chonlúin; Louise McCallom, Emily Shannon, Pia Diepman, Sophie Gleeson, Katie Walsh, Anna Roopnarinesingh, Aideen Cowley, Hannah McDermott, Caroline Healy, Aisling Nugent, Shannon Deighton, Ceara Dorman, Lorna Chow, Louise McPhillips.
ST MARY'S COLLEGE, NAAS: Aime Creaton, Meabh O'Brien, Saoirse Ferguson, áine Cummins, Marie Clarke, Chelsea Crowe, Ciara Lacey, Aoibheann Woods, Hayley Nolan, Orlaith White, Shannon Doherty, Briona Hobar, Shauna Weldon, Ciara Dunne, Emma Byrne, Alice Kinehan, Laura Lambert.