Rate splitting multiple access (RSMA) and intelligent reflecting surface (IRS) are the promising candidates and front runners for 5G and beyond wireless communication. Recent studies have proved RSMA-IRS outperforms non-orthogonal multiple access (NOMA) and enhances the quality of service (QoS). The performance of the downlink RSMA-IRS system is investigated in this paper using perfect and imperfect channel state information (CSI) conditions over the Rayleigh fading channel. Furthermore, the numerical analysis for the sum rate capacity is carriedout for the users under perfect and imperfect conditions. To assess the system’s performance, two distinct scenarios are explored. Initially, the system total capacity of each user’s common and private parts is determined without considering the channel estimate error (CEE). The total capacity for each user’s common part is 10 b/s, for private1 is 8.9 b/s, and for private2 is 6.5 b/s. After introducing CEE, the sum rate for the common part is 8.5 b/s, and the total capacity for private1 and private2 is 7.5 b/s and 6 b/s, respectively. The simulation results show that the perfect CSI, achieves enhanced sum rate than channel with CEE. however, to tolerate the imperfect scenario (CEE) optimum power allocation is determined for attaining the QoS.