Brennan made 23 league appearances for the Hoops last season but has been allowed leave Tallaght Stadium by new boss Trevor Croly, and Pat's manager Liam Buckley wasted no time in getting the former Derry City man on board.
Pat's have also secured the signature of Shelbourne winger Conan Byrne while Saints fans got a boost today with the news midfielder John Russell is staying on at Richmond Park despite offers from other clubs.
Meanwhile, managerless Limerick FC have also started building their squad, handing a new contract to defender Shaun Kelly. "I'm delighted to get my agreement sorted," Kelly said. "I'm looking forward to Premier Division football next season. Premier division football was always my goal."