  In this paper, several families of irreducible constacyclic codes over finite
fields and their duals are studied. The weight distributions of these
irreducible constacyclic codes and the parameters of their duals are settled.
Several families of irreducible constacyclic codes with a few weights and
several families of optimal constacyclic codes are constructed. As by-products,
a family of $[2n, (n-1)/2, d \geq 2(\sqrt{n}+1)]$ irreducible cyclic codes over
$\gf(q)$ and a family of $[(q-1)n, (n-1)/2, d \geq (q-1)(\sqrt{n}+1)]$
irreducible cyclic codes over $\gf(q)$ are presented, where $n$ is a prime such
that $\ord_n(q)=(n-1)/2$. The results in this paper complement earlier works on
irreducible constacyclic and cyclic codes over finite fields.
