We explore the capability of IRT-Theseus to detect dark matter through strong lensing of quasars. Quasars, though compact, are very bright and distant objects with complex structures. Their lensing by foreground galaxies is significant in cosmology. By measuring the positions of the images, the time delay between them, and the magnification ratio, we can constrain the mass distribution of the lensing galaxy, refine cosmological parameters, and understand the physics of quasars. Using updated IRT-Theseus AB magnitude data and the Quasar Luminosity Function (QLF) from the Spitzer Space Telescope, we estimate that IRT will observe approximately $N\simeq 2.7\times 10^5$ quasars with a redshift limit of $z=4.5$. Additionally, we find that one in every 2500 quasars observed by IRT will be lensed. We also examine the microlensing effect in Broad Emission Lines (BELs). With a spectral resolution of (R=400) and photometric accuracy of $5\%$, we find that IRT-Theseus can detect microlensing in BELs, enhancing our understanding of quasar physics and the mass distribution of foreground galaxies.