The aim of this paper is to study $\eta$-Ricci soliton and almost $\eta$-Ricci soliton in the context of smooth almost coKahler manifold for which Reeb vector field $\xi$ is killing and for $\xi$ belongs to $(\kappa, \mu)$-nullity distribution. For a $(\kappa, \mu)$-almost coKahler metric manifold $M$, we prove that, if $M$ is non-coKahler and $g$ is gradient $\eta$-Ricci soliton then $M$ is $\eta$-Einstein with $\lambda=0$. Next we prove that, if $g$ is an $\eta$-Ricci soliton on $M$ with $\lambda+\mu'\leq 0$, then $M$ is coKahler. Further we show that, $M$ is $\eta$-Einstein if and only if $V$ is strict infinitesimal contact transformation. Finally, we prove that, if the non-coKahler $(\kappa, \mu)$-almost coKahler manifold $M$ admits almost $\eta$-Ricci soliton with $V=\rho \xi$ or $V=Df$, then $M$ is $\eta$-Einstein. We constructed the suitable example which justifies our results.