A Katie Donnellan basket right on the buzzer had handed Mount Anville a two-point lead at the end of the first quarter.
Captain Megan Connolly then came off the bench to open up a five-point lead for Portmarnock before Rebecca Hunt replied for Mount Anville to narrow the lead, 22-19 to Portmarnock at the break.
However, with Donnellan registering her ninth point of the day, the Mount Anville team were now just a point in arrears, but that was as close as the Goatstown school could get, with Connolly, Alex Daly and Caoimhe McDonnell seeing Portmarnock pull away to claim the title.
In the girls' U-19A final, Jerome Westbrooks' Holy Faith Clontarf team lost out 59-42 to Coláiste íde & Iosef, Abbeyfeale with guard Eiblis Dillon turning in an MVP display with 23 points.
Abbeyfeale were well in charge in the first half with the outside shooting of Dillon and Tina Mangan matched on the inside by Sorcha McNulty as they led 41-21.
And while Enya Brady and Aoibhinn Byrne valiantly attempted to keep the Clontarf girls in the game in what was a vastly improved display, Abbeyfeale prevailed with Dillon's composure proving vital down the stretch.