[3] In 1967, Clifford gave a public speech on the Republican Congress in Wynn's Hotel, Dublin,[4] at a meeting of the Irish trade union group Scéim na gCeardchumann.
[5] He was an active campaigner against Irish nationalism alongside other B&ICO members including his wife Angela Clifford,[6] Jack Lane, Manus O'Riordan and Len Callendar.
[7] In the 1980s, Clifford began campaigning for the organisation of British mainland political parties in Northern Ireland.
[10] As the B&ICO became inactive in the mid-1980s, he began working through several new groups, including the Aubane Historical Society, an organisation originally intended to be a local history organisation, but later expanded into the role of opposing the "revisionist" movement in Irish history;[11] and the Ernest Bevin Society, the B&ICO's British branch.
In a piece written for The Independent, Clifford argued that Northern Catholics had no interest in a United Ireland and therefore electoral integration was the answer to the Northern conflict: "Opinion polls now confirm what one knew from experience in the Sixties, that most Catholics did not want to join the Republic.
[14][15] In the 1990s, Clifford and Lane published several books on Irish history, including Notes on Eire: Espionage Reports to Winston Churchill, 1940–2, an account of Irish writer Elizabeth Bowen's World War II intelligence reports to Britain.
The book marked an abandonment of the opposition to Irish nationalism that had characterised Clifford's earlier work.