In the aftermath of the defeat in referendum of the first Lisbon Treaty Mullen was appointed to a sub-committee on European Affairs charged with investigating the political impasse. Mullen dissented from the Sub-Committee's final report, citing the Sub-Committee's failure to take seriously potential clashes between EU law and Irish Constitutional law on socio-ethical issues as his reason for doing so. Mullen proposed that in order for the Lisbon Treaty to be passed by a referendum legally binding guarantees would have to be attached to the Treaty re-affirming sovereignty with regard to Ireland's Constitutional position on the right to life of unborn children, the family, education and religion. He also proposed that both European law and the Constitution of Ireland incorporate such limits to prevent "competence creep" in the future. The suggestion concerning legally-binding guarantees was eventually taken up by the Irish Government in its negotiations with the EU in preparation for a second attempt at a referendum. By the time of the second Lisbon referendum the Government had secured the guarantees but no other constitutional amendment was proposed as part of the plebiscite. The Treaty of Lisbon passed at the second time of asking by 67.1% in favour to 32.9% against.