The game see-sawed but Ulster eventually regained the initiative through Ryan Cobb who scored his side's opening points after 32 minutes though Cleary extended the lead to five points with a penalty moments later.
However, though Graeme Currie missed the opening conversion, he made amends with a strike on the stroke of half-time after Adam Doherty's try to give the hosts a 12-10 half time lead.
With dry conditions greeting the Leinster side for the first time in this series they produced some fine individual performances not least from Enniscorthy wing James White who beat three tackles to race clear for his side's second try of the game.
Leinster withstood some late Ulster pressure to hold on and deservedly clinch the title after previous wins over Munster and Connacht.
LEINSTER UNDER-18 (CLUB):
15: Stephen Duke (Gorey)
14: Eoghan Feane (Portlaoise) / Niall Campbell (De la Salle Palmerston) 33
13: Sean Moran (Kilkenny)
12: James Doyle (Greystones)
11: James White (Enniscorthy)
10: Fergal Cleary (Navan)
9: Ross Hynes (Arklow) / Peter Connolly (Edenderry) 33
1: Peter Dooley (Birr) CAPTAIN
2: Cian Byrne (Tullamore) / Shane Byrne (Wicklow) 59
3: Brian Cunningham (Navan) / Stephen Gardiner (Gorey) 40
4: Josh O'Rourke (De la Salle Palmerston)
5: Leon Martin (Tullamore) / Diarmaid Hickey (Kilkenny) 56
6: Eddie Earle (Gorey)
7: Stephen Keelan (Navan)
8: Killian Sweeney (Malahide) / Shane Mulvany (Greystones) 60
REPLACEMENT NOT USED: Eoghan Carron (Skerries)
Coach: Eric Miller
Assistant Coach: Dan van Zyl
Manager: Patrick Keena
Conditioning Coaches: Eugene McEntagart, Colin O'Hare
Kit Man: Noel Ferguson
Physio: Ian McGovern