Leinster survived a nervy finish at Croke Park to book their place in the Investec Champions Cup final.
James Lowe's hattrick helped them to a 20-3 lead with 25 minutes to go against Northampton in the semi final.
The English side fought back but couldn't find a way to win with the Blues hanging on 20-17.
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Leinster will find out their opponents this afternoon.
The second semi final between Toulouse and Harlequins will kick off at 3pm.
