In Case C-459/03 Commission v. Ireland the Commission brought proceedings against Ireland under Article 226 EC because Ireland had brought proceedings against the United Kingdom pursuant to Article 287 of the United Nations Convention on the Law of the Sea. (For documents on the provisional measures claim see here. There are even pictures of counsel! For documents on the arbitration case still pending before the Permanent Court of Arbitration see here). Ireland claimed that the nuclear installations at Sellafield in the United Kingdom were contaminating the Irish Sea.
The Commission claimed that by bringing such proceedings before an international jurisdiction, Ireland was in breach of Article 292 EC and 193 Euratom which conferred exclusive jurisdiction on the Court of Justice because the Law of the Sea Convention had been approved for the EC by Council Decision 98/392/EC and thus formed part of EC law as well as ratified by all member States.
The Court found in favor of the Commission. The Court held that the provisions of the Convention relied on by Ireland in the context of the dispute form part of the Community legal order. As a consequence, the Court of Justice has jurisdiction to deal with disputes relating to their interpretation or application and to determine whether a member State has complied with them. It concluded that by bringing proceedings under the dispute-settlement procedure laid down in the Convention on the Law of the Sea, without having first informed and consulted the competent Community institutions, Ireland failed to comply with its duty of cooperation under the EC and EA Treaties and in particular with their Articles 292 and 193 respectively.
See also the nice write up of the case over at the ECJ blog. There are some pictures there too.