Energy-detection receivers are appealing to IEEE 802.15.4a low data-rate networks because of their low complexity. With a reasonable energy consumption, these receivers can exploit the ranging capabilities and multipath resistance of impulse-radio UWB (IR-UWB). However, the performance of energy-detection receivers can be severely degraded by multi-user interference (MUI). One solution may be to coordinate access to the physical layer with an exclusion protocol. Unfortunately, this cannot prevent MUI due to uncontrolled activities in neighboring networks (e.g., several IEEE 802.15.4a piconets running in parallel). Hence, interference must be taken into account already in the design of the physical layer. In this paper, we present an IR-UWB receiver robust to MUI for IEEE 802.15.4a networks. Its architecture is based on energy detection. We also take into full account the different signaling structure between the preamble and the payload of IEEE 802.15.4a packets. In certain scenarios with MUI we found the packet error rate to be up to two orders of magnitude lower when compared to a traditional energy detection receiver that neglects MUI. Further, this significant performance improvement entails only a moderate increase in complexity.