Per the request to post it as an answer.
Notice that the Ad representation is a polynomial representation into $GL(Lie(G))$$\operatorname{GL}(\operatorname{Lie}(G))$. We do know that $Ad(G)$$\operatorname{Ad}(G)$ acts irreducibly, and $\Gamma$ is Zariski dense by Borel's density theorem. Hence $Ad\mid_{\Gamma}$$\operatorname{Ad}\rvert_{\Gamma}$ is also irreducible.