I have a laser sight for a gun that broke so I took it apart (it was never mounted on a gun, it just broke =/). There wasn't much in the way of shock absorption measures taken. The laser assembly was press fit into a plastic sleeve, that was inside of its metal outer case. I think the purpose for this was more for adjustment of the laser's aim than for the shock absorption. Other than that it looked like a normal laser to me.
I would be mostly worried about the high frequency vibrations that you would get from mounting it to the barrel, not so much from the recoil impulse. If I were to make an assumption based on the fact you are looking for an IR laser, and not a cheap visible one, I would say you are going to be mounting it on the rail of an AR15(or something similar)? If that is the case, especially if it is a free floating rail, I wouldn't worry about vibrations too much.
That is, unless IR lasers are more susceptible to vibrations that visible ones?