The move will certainly benefit the player but Leinster could be the real loser although they have signed Ulster and Dungannon forward Aidan Kearney, who, like Brennan, can play lock or back-row. Leinster manager Ken Ging wished Brennan all the best after hearing of his move to the French heavyweights, Brennan believed to have signed on for two years at the Stade Sept Deniers with the option of an extra season.
He joins a side whose pack include the likes of former French captain Fabien Pelous, newly-signed Test prop Jean-Baptiste Poux and Italy's Andrea Lo Cicero.
Having won 13 Irish caps between 1998-2001, Brennan was left out of the squad to tour New Zealand.
Toulouse are in French Championship semi-final action on Saturday when they face Agen in Montpellier, hoping for a repeat of their title success last season when they pipped Montferrand in the final.