One of the most decorated and vastly-experienced players in the game, the 37-year-old played over 200 times for the Brisbane Broncos in two spells, playing rugby league (between 1994 and 2000 and 2005 and 2007) where he won NRL Grand Final titles and also featured in the State of Origin series (in 1999, 2000 and later in 2005).
He became a dual rugby-code international - only the second man in history, after Bill Hardcastle, to have represented Australia in league and New Zealand in union - and in 2004 Thorn won the NPC title with Canterbury.
Commenting on the move, Leinster head coach Joe Schmidt said: "It's great to have Brad to help us through until the end of the season.
"After Leo Cullen's injury and Steven Sykes' return to South Africa we were short of options at second row, but Brad's arrival will supplement the likes of Damian Browne, Devin Toner and Mark Flanagan, as we look ahead to a demanding few months.
"Obviously Brad won't have a lot of time to find his feet, but I know the quality he brings and how keen he is to make a contribution both on and off the field."