Leinster responded in stoppage time through Stephen Smyth, but it was not enough to complete the comeback.
Leinster have begun their United Rugby Championship campaign with a narrow 27-26 defeat to the Lions in Johannesburg.
Three Kildare players were in the squad, Andrew Osbourne, Todd Lawlor, and Ciarán Mangan.
Fintan Gunne was the standout for the defending champions, scoring a first-half hat-trick as Leinster took a 18-17 lead into the break.
The Lions regained control in the second half, with a late try from Kelly Mpeku helping put the hosts seven points clear.
Leinster responded in stoppage time through Stephen Smyth, with Harry Byrne adding the conversion, but it was not enough to complete the comeback.
Leinster do leave South Africa with a losing bonus point.

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