Greene opposed the Treaty of Nice and the second referendum on it. He has acted as spokesperson on Television and Radio, for the lobby group Cóir which campaigned against the Treaty of Lisbon. He campaigned against a second Lisbon Treaty referendum and has participated in public forums on behalf of Cóir. On 20 November 2008, he led a three-person delegation from Cóir appearing at meeting of the Oireachtas sub-committee on Ireland's Future in the EU. In his opening statement, he said: