Terahertz (THz) spectrum band has garnered a lot of interest recently, due to the advances in supporting radio electronics and the existence of hundreds of Gigahertz of available spectrum. Therefore, it is considered as one of the key enablers for the upcoming sixth generation (6G) wireless networks, which target to provide Terabit per second communication services. However, the propagation loss due to the high carrier frequency, obstacles, and the atmospheric attenuation in this frequency band need to be compensated to enable usable coverage ranges. In this paper, we investigate the potential of using Intelligent Reflecting Surface (IRS) for enhancing the coverage of Terahertz communication. We conduct a case study for an indoor cellular communication scenario with multiple Transmission and Reception Points supported by IRS. The numerical result show that the IRS can enable significant coverage extension. Besides, by optimizing the frequency resource allocation and IRS selection jointly considering the frequency-dependent atmospheric absorption, the throughput of underserved users can be improved significantly.