Brennan - who left Leinster for south-west France two-and-a-half years ago - formed a makeshift lock partnership with youngster Romain Millo-Chluski, due to captain Fabien Pelous' knee injury.
And the 31-year-old "Barnhall Bruiser" pulled out another big European performance as Toulouse - thanks to tries from Finau Maka, Frederic Michalak and a crucial one from scrum-half Jean-Baptiste Elissalde, who also kicked 12 points - won through to a record fourth final since their first success in 1995/96.
Brennan - who started as lock in last year's agonising 27-20 final loss to Wasps - said afterwards: "English teams are alway hard to play against, but we had enormous respect for Leicester and their past record in the competition.
"The pain of losing last year to Wasps is still there for a lot of our players and I think that inspired the performance out there," added the Leixlip man.
"It might have been a different outcome had Leicester seized on some of their opportunities early on in the first half but I think we deserved it in the end."
Brennan earned the last of his 13 Ireland caps in November 2001, coming on as a second half replacement in the 35-8 Lansdowne Road win over Samoa.
Guy Noves' triumphant Toulouse side now face Stade Francais - 20-17 conquerors of Biarritz Olympique - in the second all-French Heineken Cup final in two years at Edinburgh's Murrayfield Stadium on Sunday, May 22 (kick-off 3.00pm).
