We report a detailed theoretical and numerical investigation of the sensing mechanism and performance of plasmonic fiber-optic sensors using group IV transition metal nitrides. We first compared the plasmonic properties of hafnium nitride (HfN), zirconium nitride (ZrN), and titanium nitride (TiN) to gold (Au) as a conventional plasmonic material and designed two different plasmonic fiber-optic sensing platforms using side-polished single mode fibers and few mode fibers (FMFs). Using the finite element method, we demonstrated that the sensing mechanisms in the proposed sensors are based on the interplay between fiber and plasmonic modes and variation of resonance wavelength depending on analyte refractive index. We show that HfN and ZrN can considerably outperform Au in the visible region as alternative cost-effective plasmonic materials for the design of fiber-optic sensors with more than three times larger sensitivity and a sensing figure of merit of almost eight times that of Au. In particular, HfN-coated FMF sensors can demonstrate an average linear sensitivity of 6140 nm/RIU, a maximum sensitivity of 8200 nm/RIU, and an average (maximum) figure of merit of 133 (201) for analyte refractive indices between 1.33 and 1.38. We show that the figure of merit of the proposed simple side polished HfN coated FMF is more than four times larger than that of the previously reported similar fiber-optic sensors. The results show the potential of group IV transition metal nitrides, particularly HfN and ZrN, to replace Au in various fiber-optic applications including monitoring marine environments, medical diagnosis, and biosensing. Graphic Abstract