"$S$ is the unique irreducible $\operatorname{End}(S)$-module"

Question

Let $S$ be a finite-dimensional vector space. Then $S$ is an irreducible $\operatorname{End}(S)$-module. Furthermore I was told that $S$ is the only irreducible $\operatorname{End}(S)$-module. I guess that this means that if $E$ is an irreducible $\operatorname{End}(S)$-module, then there is a unique isomorphism of modules, i.e. $\operatorname{Hom_{\operatorname{End}(S)}}(S,E)$ contains a unique invertible element. Since I have no prior training in algebra, I am hoping that someone can either name a reference or outline the proof.