Is $\displaystyle f(x)=x^3+15x^2+8x+6$ irreducible in $\displaystyle \mathbb{Z}[x]$, $\displaystyle \mathbb{Z}[[x]]$, $\displaystyle \mathbb{Z}_5[x]$?

I've tried doing the $\displaystyle \mathbb{Z}[x]$, but we've only mentioned Eisenstein's criterion and I can't apply it here.

Thank you.