Mac Cába is educated at Keash national school and Summerhill College, Sligo. He wins a scholarship to St. Patrick’s College of Education, Drumcondra, Dublin, qualifying as a primary schoolteacher. He later obtains a diploma in education from University College Dublin (UCD) and is appointed principal of Drumnagranchy national school in County Sligo in 1907.
Mac Cába is elected as a Sinn Féin Member of Parliament (MP) for the constituency of Sligo South at the 1918 general election. In January 1919, Sinn Féin MPs refuse to recognise the Parliament of the United Kingdom and instead assemble at the Mansion House in Dublin as a revolutionary parliament called Dáil Éireann. Mac Cába, however, does not attend as he is in prison at the time.
At the 1921 Irish elections, Mac Cába was re-elected for Sligo–Mayo East. He supports the Anglo-Irish Treaty and votes in favour of it. He is again re-elected for Sligo–Mayo East at the 1922 general election, this time as pro-Treaty Sinn Féin Teachta Dála (TD). During the Treaty debate he asserts that the counties of Ulster which comprise “Northern Ireland” can never be incorporated into an Irish Republic while the British Empire is what it is.
At the 1923 general election, Mac Cába is elected as a Cumann na nGaedheal TD for Leitrim–Sligo. He resigns from Cumann na nGaedheal in 1924 because of dissatisfaction with government attitude to certain army officers and joins the National Party led by Joseph McGrath.
Mac Cába resigns his Dáil seat in March 1925 along with several other TDs, and at the resulting by-election on March 11, 1925 Cumann na nGaedheal candidate Martin Roddy wins his seat. He does not stand for public office again and returns to his post as a schoolteacher.
In the 1930s Mac Cába is involved with the short-lived but widely followed Irish Christian Front, serving as the organisation’s secretary and announcing its creation to the public on August 22, 1936. He is also member of the Blueshirts during this period and later the Irish Friends of Germany during World War II, a would-be Nazi Collaborator group in the event Germany invades Ireland. He chairs their meetings, denies the group is a fifth column and expresses the belief that a German victory would lead to a United Ireland. He is interned in 1940–1941 because of his pro-German sympathies, which he claims results from the desire to “see the very life-blood squeezed out of England.”
Mac Cába dies in Dublin on May 31, 1972, leaving his wife, son, and three daughters. There is a bronze bust of him in the headquarters of the Educational Building Society, Westmoreland Street, Dublin.