This paper introduces an innovative hybrid free-space optical (FSO) and sub-Terahertz (Sub-THz) communication system that improves reliability through the use of intelligent reflecting surfaces (IRS) and a dual-threshold switching mechanism. The considered system tackles significant problems, including atmospheric turbulence, propagation loss, and alignment errors, by utilizing IRS to enhance connection stability. The dual-threshold method, in contrast to traditional single-threshold switching, reduces frequent link transitions, resulting in a more stable and efficient communication system. The performance analysis indicates that the suggested architecture substantially surpasses conventional IRS-assisted FSO systems and single-threshold hybrid models in terms of outage probability. Numerical results offer critical insights for establishing IRS parameters, switching threshold design, and assuring resilient communication in forthcoming 6G networks.