The Exiles, who have won just one Premiership away game this season, were soundly beaten by the Saints, shipping six tries in a 40-14 defeat.
Smith’s men had gone in 14-9 down at half-time at the Gardens, but succumbed after the break as Northampton turned the screw.
The result ensures Irish stay just five points clear of basement team Sale Sharks with five games left to play.
And Smith was eager to wipe Saturday’s showing from his memory as quickly as possible.
“They were coming downhill towards their own in the second half and they ran riot,” he said. “It’s very disappointing.
“We had a couple of scoring chances in the first half and we really needed to take them. We were in their red zone and we only came away with three points.
“We knew we had to take them by the jugular and we had to at least score twice in our trips to their 22.
“At 14-9 at half-time you’re in the contest, but the second half is one we’d rather forget.”
Saints were ruthless during a second half that saw them score four tries, with wing Jamie Elliott going on to complete his hat-trick.
And Smith was disappointed with the way his side surrendered possession on a staggering number of occasions.
“They scored just before half-time and the drove it home in the second half,” he added.
“We turned the ball over 23 times and we don’t do that in a season, but we did it on Saturday.”