A run of four unanswered tries either side of half-time saw United Rugby Championship leaders Leinster beat Cardiff 42-24.
Despite the red card, the visitors continued to be the better side with Cardiff having to survive sustained late pressure in order to pick up two points and cement third position in the league.
,Cardiff had to settle for a bonus point after kicking off their European Challenge Cup campaign with a thrilling 37-26 defeat in Lyon.
Cardiff will feature seven new signings in their opening pre-season friendly against Ealing Trailfinders tomorrow at the Trailfinders Sports Club.
Cardiff were in contention throughout but two tries from international prop Nicky Smith and others from Justin Tipuric and Luke Morgan proved just enough.
England flanker Lewis Ludlam is set to return from a two-month absence tomorrow when he captains Northampton Saints against Newcastle Falcons at Franklin's Gardens in the Gallagher Premiership.
The home side were second best in terms of possession and territory but were the more clinical to achieve a morale-boosting bonus-point win.
Cardiff have signed former Bath winger Gabriel Hamer-Webb on a short term deal as injury cover for Josh Adams and Owen Lane.
Bath have confirmed that they will keep Russian international Valeriy Morozov, who recently joined the side following the demise of Worcester Warriors.
