Intelligent reflecting surface (IRS) can actively control wireless propagation channels using multiple reflective elements and is a promising technique to overcome performance degradation caused by blockages. To analyze the performance of IRS systems adopted for ultra-high frequency bands, such as 28 GHz or above, carefully considering the effect of the blockage on performance evaluation is necessary. In this research work, we analyzed the system capacity of the IRS with blockages by using blockage probability derived from the density and velocity of the blocker. Numerical results showed that, for a relatively low blocker density, such as 0.1 blockers per square meter, IRS height is closer to the mobile station (MS) antenna height. Conversely, with an increase in blocker density up to 0.3 blockers per square meter, the IRS height increased owing to blockage reduction.