Schmidt had the luxury of being able to name Isaac Boss and Seán Cronin, both members of Declan Kidney's World Cup squad, along with Luke Fitzgerald, who failed to make the cut, in his squad.
But it was Northampton who soon had the upper hand with Stephen Myler opening the scoring with an 11th minute penalty while only a superb tackle from Andrew Conway preventing Noach Cato crossing.
Fitzgerald looked bright with ball in hand but two further penalties from Myler gave the visitors a 9-0 halftime lead.
Saints were on top at the start of the second half and deservedly scored the game's first try through Paul Diggin's intercept touchdown with Ryan Lamb converting.
That finally stung the hosts into life and they got back into the game when Harris-Wright smashing his way over from close range.
Matt Berquist converted and was soon repeating the process when Carr capped a fine performance by sprinting in from long range but a Lamb penalty just moments before made the game save for Saints.
Leinster meanwhile have to pick themselves up for their opening RaboDirect PRO 12 game at Scarlets.
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