Young back rower Dominic Ryan has also signed a fresh two-year deal having featured in 22 games so far this season. The Ireland underage international has also scored four tries in both the Magners League and Heineken Cup to date.
Meanwhile, centre Brendan Macken and out-half Ian Madigan have extended their current deals with Leinster, while promising half-backs John Cooney and Ian McKinley have both signed their first professional deals with the province.
Galway native Damien Browne, who is 30, is a former Ireland Under-21, Connacht and Northampton Saints lock who joins from French Top 14 club Brive, having featured in 30 games so far this season.
Former Leinster Academy graduate Fionn Carr, meanwhile, returns to his home province having impressed for Connacht in recent seasons.
The Newbridge winger has also represented Ireland at Schools, Under-19, Under-21 and 'A' levels.
Commenting on the new deals, Leinster head coach Joe Schmidt said: "We are delighted to announce the signings of two Irish-qualified players in Damien and Fionn and we believe that both players have the potential to make an impression over the coming season.
"I know how enthusiastic both players are about linking up this summer, in Fionn's case his arrival will be a homecoming of sorts as he is a graduate from the Leinster Academy.
"Richardt has been a key performer in the Leinster front row this year and his contract extension is a further boost in addition to other recent contract extensions.
"With the extensions and new contracts for the younger players, it is important that we continue to retain our best underage talent from the Leinster Academy."