The real-time implementation of emerging technologies faces diverse challenging environments. To ensure reliable transmission, robust communication strategies are necessary. In this regard, transmit antenna diversity in the form of orthogonal space-time block coding (OSTBC) has been explored in non-orthogonal multiple access (NOMA) systems. This has appreciably improved the reliability and throughput of the wireless communication system. To further boost the performance outcomes, we propose the incorporation of intelligent reflecting surfaces (IRS) in downlink OSTBC-NOMA systems. The advantages of this hybrid scheme motivate its implementation in real-world environments. To examine this strategy, we develop a detailed statistical framework for the proposed system over Nakagami \(-m\) fading channels. We derive the probability density function (PDF) and cumulative distribution function (CDF) of the users by leveraging the Laguerre series approach. Based on these derived expressions, we investigate the performance of the proposed system and derive new closed-form expressions of average bit-error rate (ABER), average channel capacity (ACC), and outage probability (OP). Unlike prior works, this is the first study which combines IRS, OSTBC, and NOMA technologies, exploiting Laguerre series to derive closed-form expressions. We also discuss the convergence and complexity analysis of the proposed approach. We examine the system at high signal-to-noise ratio (SNR) and consequently obtain the diversity order of the users. We study the influence of reflecting elements, successive interference cancellation (SIC) errors and fading parameters on the derived performance parameters of the users. We also compare the obtained results with benchmark technologies, revealing the superior performance of the proposed system. To validate the derived expressions, Monte Carlo simulations are executed which match the obtained results.
