In this work, we consider the performance analysis of the uplink cellular networks with free space optics (FSO) backhauling. The user equipment (UE) communicates with the nearest Base Station (BS) in first slot while in second slot, the BS converts the received radio frequency (RF) signal into FSO pulse and transmits to the data center. We adopt the Rayleigh fading for the uplink channels while the FSO backhaul encompasses the turbulence-induced fading which follows the Málaga distribution, the weather pathloss, and the pointing errors fading which is distributed following the generalized Beckmann model. Further, we will compare the performances when the BS behaves as either a decode-and-forward (DF) relay or an intelligent reflecting surface (IRS). Next, we will propose an optimal design of the phase shifters of the IRS to minimize the interference and improve the rate to beat relaying. Capitalizing on this framework, we will derive the system performance metrics such as the coverage probability as well as the spectral efficiency. Focusing on high SNR, we will obtain the diversity gain to get engineering insights into the system performance and limitations. Finally, the analytical results are confirmed by Monte Carlo simulations.