  We report on systematic evolutions of antiferromagnetic (AFM) spin
fluctuations and unconventional superconductivity (SC) in heavy-fermion (HF)
compounds CeRh$_{1-x}$Ir$_{x}$In$_5$ via $^{115}$In
nuclear-quadrupole-resonance (NQR) experiment. The measurements of nuclear
spin-lattice relaxation rate $1/T_1$ have revealed the marked development of
AFM spin fluctuations as a consequence of approaching an AFM ordered state with
increasing Rh content. Concomitantly the superconducting transition temperature
$T_{\rm c}$ and the energy gap $\Delta_0$ increase drastically from $T_{\rm c}
= 0.4$ K and $2\Delta_0/k_{\rm B}T_{\rm c} = 5$ in CeIrIn$_5$ up to $T_{\rm c}
= 1.2$ K and $2\Delta_0/k_{\rm B}T_{\rm c} = 8.3$ in
CeRh$_{0.3}$Ir$_{0.7}$In$_5$, respectively. The present work suggests that the
AFM spin fluctuations in close proximity to the AFM quantum critical point are
indeed responsible for the onset of strong-coupling unconventional SC with the
line node in the gap function in HF compounds.
