The home side scored three tries inside the opening 25 minutes to set up the 22-15 win.
The Exiles' director of rugby told Sky Sports News: "I am very disappointed with our start, they were much more physical and more urgent than us as we lacked any intensity.
"After that our fighting qualities showed through and if we had kicked our goals it would have been an even game. It was always our plan to take off some of our key players at half time as we have to look after them in readiness for the big games that lie ahead.
"The conditions became a lottery, but in the end we turned the ball over very easily and weren't good enough."
Irish are yet to win a match in the competition this season.