In 1943 Sheehy-Skeffington was expelled from the
Labour Party, the reasons for which were often disputed. The Irish historian
Diarmaid Ferriter suggests he was expelled for engaging in a public spat with a Catholic priest over the nature of socialism. Other sources suggest communists in Dublin, who had entered the Labour Party under the doctrine of
entryism, had ousted him because they perceived him to be veering towards
Trotskyism. Others, such as
Noël Browne, suggested he had been expelled for "simply being too liberal". In 1954 Sheehy-Skeffington moved into formal parliamentarian politics when was elected as a member of the
8th Seanad by the
Dublin University constituency. He was re-elected in 1957, but lost his seat in 1961. He was returned to the
11th Seanad in 1965 and was re-elected for a final time in 1969. In the Seanad he was known as a champion of
civil liberties and an opponent of
authoritarianism. Among many issues, he campaigned for an end to
corporal punishment in Irish schools, an end of control by the Catholic Church of government-funded schools, stood against censorship, denounced terrorism, championed
women's rights and opposed
Apartheid. On matters of the "Irish Question", Sheehy-Skeffington cited
James Connolly's analysis and suggested both the Republic of Ireland and Northern Ireland each needed political reform first, then merged, rather than the other way around. Still citing Connolly, he also voiced the view that Irish independence from the United Kingdom meant nothing if all it amounted to was to change the colour of the flag flying over its institutions, and instead, the change must also be a meaningful change in conditions for the people of Ireland. He also noted that at least 4 of the 6 counties which made up Northern Ireland were made up of solid majorities of Protestant
Unionists who he argued could not be coerced, by violence or otherwise, into the Irish state and that Republicans needed to accept this reality and alter their tactics accordingly, with more emphasis given to social conditions. He was an
atheist and helped set up the
Humanist Association of Ireland. He was also a co-founder and active member of the Irish Association for Civil Liberty, which he co-founded in 1948 with the writer
Seán Ó Faoláin and others. In 1958, activist
Peter Tyrrell wrote to Sheehy-Skeffington about the abuse he experienced in
St Joseph's Industrial School in Letterfrack. This began a correspondence between them, lasting until Tyrrell's death, in which Tyrrell, at Sheehy-Skeffington's encouragement, wrote his autobiography. It was published in 2006 as
Founded on Fear and helped expose the brutal conditions in
Irish industrial schools, and in Letterfrack in particular. After Tyrrell committed suicide in 1967 the only clue to his identity was a card addressed to Sheehy-Skeffington. On the eve of Sheehy-Skeffington's death the
Arms Crisis was beginning to play out and one of Sheehy-Skeffington's final acts was to send a letter to
Taoiseach Jack Lynch expressing support for his actions taken against
Charles Haughey and
Neil Blaney, dismissing them from cabinet. ==Death and legacy==