Errr...what kind of lens? Lenses can either focus light or disperse it. Additionally, some lenses are good at passing infrared light and some lenses heavily attenuate it. It's not just a piece of glass!
It's a double convex lens. I don't have any information on whether it works with IR well, but it works with a red LED perfectly.
There's also the focal length of the lens to consider
I have the LED exactly the focal length away from the lens (in this case it is 15 cm), and I did make sure that it was focusing well using a red LED first. I could see the red LED on a white piece of paper in the day time (my windows were closed and lights were off but light from outside still gets in anyway) from 10 feet away.
What specific IR sensor are you using?
The sensor I'm using is the TSOP4856 (Datasheet: http://www.vishay.com/docs/82090/tsop48xx.pdf