EU law normally override EU member state national law. The EU court is the final interpreter on EU law.
The thinking about lower courts unappealable ability to ask the EU court for what the EU law is, that a higher or highest national courts should never be able to suppress a question from reaching the EU court.
Hence, if the Supreme Court in a member state would give a final ruling on a topic, any lower court that thinks the Supreme Court were wrong on points of EU law, may always in a following case ask the EU court for final guidance on what EU law actually means (instead of just following the Supreme Court ruling). The Supreme Court would in that instance have no means of preventing that from happening.