D'Arcy, who toured New Zealand with the Lions four years ago, joins fellow Leinster players Luke Fitzgerald, Jamie Heaslip, Rob Kearney and Brian O'Driscoll in the now 38-man party.
The 29-year-old's addition take the total number of Magners League players in the Lions squad to 27, following Leigh Halfpenny's arrival in South Africa yesterday and James Hook's late call-up just before departure for the Republic.
"Gordon has been called up due to the fact that we have had a few injury concerns in the backs," explained McGeechan.
"While the injuries are being treated by the medical team we felt it prudent that we call up Gordon to provide us with extra cover in midfield."
Having already seen Magners League players Tomas O'Leary, Tom Shanklin, Jerry Flannery and Alan Quinlan ruled out of contention before the party even left British and Irish shores, McGeechan is keen to cover every possibility when it comes to any further disruptions.
"Due to the short nature of this tour we have to be prepared for every eventuality," added McGeechan.
"An extra back at this stage will provide the extra depth to cover the existing injuries plus any further injuries we may encounter.
"Gordon showed in the European Rugby Cup Final and the recent Barbarians match against England that he is back to his best. He was on our reserve list and we had no hesitation in calling him up to the squad as he is an extremely experienced player who has played 41 times for his country.
"He was also a British & Irish Lion on the 2005 tour to New Zealand where he played six provincial matches. He earned a Lions' Test cap in the match against Argentina in Cardiff prior to that tour. His partnership with Brian O'Driscoll for Leinster and Ireland is a proven one and can only benefit the Lions' squad here in South Africa."
D'Arcy will arrive in South Africa this Thursday, at which point he will meet up with the squad in Johannesburg ahead of Saturday's clash with the Cheetahs.
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