Michael O'Riordan (Irish: Mícheál Ó Ríordáin, 12 November 1917 – 18 May 2006) was a leader of the Communist Party of Ireland and also fought with the Connolly Column in the International Brigades during the Spanish Civil War.
O'Riordan was born at 37 Pope's Quay, Cork City, on 11 November 1917. He was the youngest of five children. His parents came from the West Cork Gaeltacht of Ballingeary-Gougane Barra. Despite his parents being native speakers of the Irish language, it was not until O'Riordan was interned in the Curragh camp during the Second World War that he learnt Irish, being taught by fellow internee Máirtín Ó Cadhain who went on to lecture in Trinity College, Dublin.
As a teenager, he joined the republican youth movement, Fianna Éireann, and then the Irish Republican Army. The IRA at the time was inclined towards left wing politics and socialism. Much of its activity at the time concerned street fighting with the quasi-fascist Blueshirt movement and O'Riordan fought Blueshirt fascism on the streets of Cork City in 1933–34. O'Riordan was friends with left-wing inclined republicans such as Peadar O'Donnell and Frank Ryan, and in 1934, he followed them into the Republican Congress – a short-lived socialist republican party.
O'Riordan joined the Communist Party of Ireland in 1935 while still in the IRA and worked on the communist newspaper Socialist Voice. In 1937, following the urgings of Peadar O'Donnell, several hundred Irishmen, mostly IRA or ex-IRA men, went to fight for the Spanish Republic in the Spanish Civil War with the XVth International Brigade. They were motivated in part by enmity towards the 800 or so Blueshirts, led by Eoin O'Duffy who went to Spain to fight on the "nationalist" side in the Irish Brigade. O'Riordan accompanied a party led by Frank Ryan. In the Republic's final offensive of 25 July 1938, O'Riordan carried the flag of Catalonia across the River Ebro. On 1 August, he was severely injured by shrapnel on the Ebro front. He was repatriated to Ireland the following month, after the International Brigades were disbanded.