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.
"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).