In this letter, a non-orthogonal multiple access (NOMA) scheme is employed for irregular repetition slotted ALOHA (IRSA). Specifically, packet replicas are transmitted with discrete power levels, which are pre-determined by the NOMA scheme. In this case, most packet collisions can be resolved in the power domain, contributing to a much lower packet loss rate. Density evolution analysis is formulated and the degree distributions are optimized for different numbers of power levels. The simulation results validate our analysis and show that the proposed scheme can outperform existing IRSA schemes.