The Court of Justice of the European Union is the supreme court for all Northern Ireland Protocol issues.
THE COURT OF JUSTICE OF THE EUROPEAN COMMUNITIES (CJEC) The Court of Justice comprises as many judges as there are Member States. The number of Advocates-General remains the same (eight), but the Council may at any time decide unanimously to increase their number. [...] The Court of Justice, as the supreme judicial institution of the Union, retains competence for other judicial actions on fundamental questions for the Community order and carries out this mission by way of questions referred by the national jurisdictions for a preliminary ruling. However, the Treaty provides that the Statute may empower the Court of First Instance with preliminary competence in certain areas.