In this letter, we initially present a formulation of Received Signal Strengths (RSS) in a scenario involving one source (or emitter), one Intelligent Reflecting Surface (IRS), and a Wireless Sensor Network (WSN). To determine the source location based on RSS measurements, we carefully select IRS phases to simplify the relationship between them. Subsequently, we solve the resulting rather simple nonlinear equations using a Least Squares (LS) approach. The solution involves an initial estimation through a course search, followed by a Steepest-Descent (SD) recursion. Simulation results highlight the superior performance of SD compared to course search and some state-of-the-art RSS localization techniques in the literature.