We consider a multi-user impulse radio UWB physical layer in a multipath environment. We propose a fast and efficient method to compute the conditional bit error rate (BER), given some realizations of the channels from source/interferer to destination, and of delay differences. Our motivation is packet level simulation of large scale or dense impulse radio UWB networks. The conditional BER is used in a packet level simulator with block fading channel assumption to sample packet transmission error events. However, due to the timescale difference between physical layer events and network events, a pulse-level simulation of the BER in a realistic multipath channel environment is infeasible. Our solution is based on a novel combination of large deviation and importance sampling.