The article presents the legal framework of EUNAVFOR MED IRINI, a naval force established by the EU to contribute to the implementation of the arms embargo on Libya by UNSC Resolution 1970 (2011). The legal basis of IRINI is to be found both in the EU Council’s Decision establishing it, and the UNSC Resolution it aims to implement. The coexistence of two distinct sets of rules, the collective security mechanism (Ch. VII UN Charter and the EU law, poses legal challenges. The main issues that have arisen from the operation of IRINI and that will be studied in this article are the interpretation and application of the rule of the exclusive flag State jurisdiction and the issue of the sovereign immunity of vessels.