D'Arcy, who toured New Zealand with the Lions four years ago but did not win a Test cap, will arrive in South Africa this Thursday and will immediately join up with his new team-mates in Johannesburg.
"Gordon has been called up due to the fact that we have had a few injury concerns in the backs," explained head coach McGeechan, who is currently putting the final touches to preparations for tonight's clash with the Golden Lions at Coca Cola Park.
"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."
D'Arcy was a member of Ireland's Grand Slam winning squad earlier this season having recovered from a long-standing injury and will join the Lions in confident mood following his role in Leinster's Heineken Cup triumph late last month.
McGeechan has already lost Tomas O'Leary, Tom Shanklin and Jerry Flannery to injury and Alan Quinlan to suspension and the Scot is now 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's arrival in the Republic comes two days before the Lions' third tour match against Super 14 side the Cheetahs in Bloemfontein.