Kirchner and Kearney will be team-mates from next term, though, after the 28-year-old Springbok signed a two-year deal at the RDS.
"We have been in talks with Zane and his agent since November and, although it is very disappointing to lose his services, we wish him well with the new phase in his career," said Xander Janse van Rensburg, the high performance manager at Kirchner's current club, the Blue Bulls.
"From our side, with coach Frans Ludeke very involved and in regular consultation with Springbok management, we tried our best to keep the player in Pretoria but could in the end not compete with the exchange rate and the expectations of Zane and his agent."
Kirchner has 24 Springbok caps to his name having made his debut when the Lions won 28-9 at Coca Cola Park to ensure their most recent adventure ended on a high.
The former Griquas star, who was part of his country's last Test-match squad against England in November of last year, will fill the void left at Leinster by utility back Isa Nacewa after the Fijian decided to call time on his career at the end of the current campaign.
"Zane Kirchner is a quality international player and we believe that he will add genuine value to the Leinster squad over the next few years," said Leinster head coach Joe Schmidt.
"He's well balanced, fast and skilful and has featured for the Springboks in their last nine internationals.
"At 28, he is ideally placed to contribute to team performance while also passing on his experience to some of our younger outside backs."
No date has yet been set for Kirchner's arrival in Ireland but the Blue Bulls are expecting he will ask to be allowed to join Leinster close to the start of the season despite the fact that his contract runs until October 31.
"We understand that an earlier release might be requested, but it is up to the board to approve that or not," added van Rensburg.
Kearney signed a new contract to remain with Leinster for a further two years in early February, meaning he will be staying with the Heineken Cup holders until the end of the 2014/15 season at least.