The judges of the Supreme Court have frequently relied on Clarke to write judgments for the court in cases involving public law. He delivered the judgments of the court in "Kerins v. McGuinness" and "O'Brien v Clerk of Dail Éireann" which clarified the law of parliamentary privilege in Ireland. In 2018 he wrote an opinion on behalf of a seven-judge panel which held that references the "unborn" under the Eighth Amendment of the Constitution of Ireland referred to an unborn child and the rights conferred upon an unborn child were confined to that section. His judgment in "Friends of the Irish Environment v Government of Ireland" "recharacterized" the approach of the Supreme Court to unenumerated rights, instead considering them "derived rights" under the Constitution of Ireland.