  Let $M$ be compact negatively curved manifold, $\Gamma =\pi_1(M)$ and
$\tilde{M}$ be its universal cover. Denote by $B =\partial \tilde{M}$ the
geodesic boundary of $\tilde{M}$ and by $\nu$ the Patterson-Sullivan measure on
$X$. In this note we prove that the associated unitary representation of
$\Gamma$ on $L^2(B,\nu)$ is irreducible. We also establish a new rigidity
phenomenon: we show that some of the geometry of $M$, namely its marked length
spectrum, is reflected in this $L^2$-representations.
