Metal nanoclusters have several applications in biological processes, medicine, cancer therapy, catalysis, etc. Iridium (Ir) nanoclusters exhibit excellent detection behavior compared to their bulk material. This work includes a deep insight into the interaction of Ir nanoclusters of four atoms (Ir4) with amino acids and the analysis of Ir-amino acid (Ir-AAc) complexes. UV-visible spectroscopy of the Ir4 nanocluster, amino acids, and their complexes was discussed as a way to detect amino acids with the help of the Ir4 nanocluster. In UV-visible analysis, the UV-visible peak of phenylalanine (Phe) appeared at 204 nm with an excitation energy of 6.02 eV with a 0.0516 oscillator strength. Meanwhile, only in the Ir4 nanocluster-phenylalanine (Ir-Phe) complex, the UV-visible peak was observed at 661 nm with an excitation energy of 1.87 eV and oscillator strength of 0.0051. This peak was observed due to the transition from HOMO-1 to LUMO+3. In the other complexes, no UV-visible peaks are observed. Thus, the results predict that the Ir4 nanocluster can be used in the detection of Phe via UV-visible spectra.