  In this paper we develop a method to describe perturbatively the entanglement
entropy in a simple impurity model, the interacting resonant level model
(IRLM), at low energy (i.e. in the strong coupling regime). We use
integrability results or the Kondo model to describe the infra-red fixed point,
conformal field theory techniques initially developped by Cardy and Calabrese
and a quantization scheme that allows one to compute exactly Renyi entropies at
arbitrary order in $1/T_B$ in principle, even when the system size or the
temperature is finite. We show that those universal quantities at arbitrary
interaction parameter in the strong coupling regime are very well approximated
by the same quantities in the free fermion system in the case of attractive
Coulomb interaction, whereas a strong dependence on the interaction appears in
the case of repulsive interaction.
