Intelligent reflecting surfaces (IRSs) are an emerging technology that can be used to alleviate the requirements of a strict line of sight in free-space optical communication (FSO) systems. This paper investigates the performance of an IRS-assisted FSO system under the conditions of strong atmospheric turbulence fluctuations, taking into account the impact of the spatial jitter at the transmitter and the IRS. A closed-form for the bit-error rate (BER) has been derived. The impact of the strong atmospheric turbulence is characterized using the K-distribution. The obtained results demonstrate that the performance depends on the system configuration as well as the transmitter and IRS jitter. Furthermore, we show that optimal placement of the IRS can improve the BER.