The striker makes a return to Richmond Park after a sojourn which saw him line out for Monaghan United and Scunthorpe United before being released
The 20-year-old, who can play as a forward or midfielder, joins the Richmond Park club for a second time after his release by English League One outfit, Scunthorpe United.
Keegan previously played for the Saints in 2011, before joining Monaghan United in the summer of that year. He remained at Gortakeegan until mid-way through 2012, when, after the club's demise, he joined Scunthorpe United.
The Dublin native made just four appearances for the Iron before being released by manager Brian Laws, who explained that the player was 'not quite at the level' required.
"There's going to be a partial settlement for the two boys (Mark Hughes and Jordan Keegan) so they can move on," Laws told reporters. "It's unfortunate, but they're just not quite at the level we are and where they need to be.
"I think it's to their detriment if you let them hang around when they've no chance of playing. At least we can move them on and release a little bit of capital."