The 19-year-old was a mainstay of last season’s double winning side but released along with fellow defenders Matty Hughes and Stephen O’Donnell.
Hughes is currently on loan at Colwyn Bay from Fleetwood Town while O’Donnell is a regular at Partick Thistle with Brennan having to go further afield to make his mark.
The defender from Wishaw went over on trial to Slovakia in the new year and did enough to be offered a contract.
In an emotional piece on his Facebook page he wrote: “Since playing for Celtic from the age of 8 and leaving at 18 it was emotional to leave the club I love.
“My head was a mess, the effect hit me hard. My head went and I struggled to deal with real life but no matter what the struggle no faith was lost, if anything it made me stronger.
“The sh** I had to do, for the people who know me they’ve seen my journey and it was no means easy… for the doubters thanks!
“Bye Scotland, I’m signing for Spartak Trnava who played in Europe last year and are two points off a Champions League place, things are looking up for me now- thanks to everyone who believed in me- I’m back.”
Trnava are currently sitting in third place in the Slovakian league just a point behind leaders Zilnia.
The Slovakian league resumes on February 25 with Trnava away to Dukla.
Best of luck to double winner Keiran as he starts a new chapter in his career.
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