Energy-detection (ED) receivers can take advantage of the ranging and multipath resistance capabilities of impulse-radio ultra-wideband (IR-UWB) physical layers at a much lower complexity than coherent receivers. However, ED receivers are extremely vulnerable to multi-user interference (MUI). Therefore, the design of IR-UWB ED architectures must take MUI into account. In this paper, we present the design and evaluation of two complementary algorithms for reliable and robust synchronization of IR-UWB ED receivers in the presence of MUI: 1) power-independent detection and preamble code interference cancellation (PICNIC) and 2) detection of start-frame-delimiter through sequential ratio tests (DESSERT). PICNIC addresses packet detection and timing acquisition while DESSERT focuses on start-frame-delimiter (SFD) detection. Both algorithms are evaluated with the IEEE 802.15.4a IR-UWB physical layer, standardized for low data-rate networks. The performance evaluation with extensive simulations show that our algorithms outperform nonrobust synchronization algorithms by up to two orders of magnitude in the presence of MUI.