Let $\mathbb{F}_q$ be a finite field of order $q$ and integer $n\ge 1$. Let $\ell$ be a prime such that $\ell^k|(q-1)$ for some integer $k\ge 1$ and $μ$ be an element of order $\ell^k$ in $\mathbb{F}_q$. In this paper, we determine the weight distributions of all irreducible $μ$-constacyclic codes of length $\ell^n$ over $\mathbb{F}_q$. Explicit expressions for the generator polynomials and codewords of these codes are also obtained.