Tomás's father Robert Paul Gill, an engineer and architect,[3] also stood unsuccessfully for election on a number of occasions.

A qualified Accountant, Mac Giolla was employed by the Irish Electricity Supply Board (ESB) from 1947 until he went into full-time politics in 1977.

In 1961, he married May McLoughlin who was also an active member of Sinn Féin as well as of Cumann na mBan, the women's section of the IRA.

Initially, Mac Giolla's approach to his former comrades in the Provisional movement was one of conciliation, though over time, bitterness emerged.

As violence in Northern Ireland escalated, Mac Giolla reiterated that the Official IRA (OIRA) favoured mass popular resistance, but would resort to "retaliatory" military action when deemed necessary.

[2] After the OIRA killed Unionist Northern Ireland Senator John Barnhill, Mac Giolla initially blamed British agents.

The question of how far the OIRA should go in using armed force was one of the issues that led to the ceasefire in May 1972, which Mac Giolla fully supported.

By 1978, Mac Giolla was asserting that "the Provos are engaged in a war against the Irish people … can anyone say that the atrocities of the infamous Black and Tans were any worse?"

[5] In November 1973 Tomás Mac Giolla addressed the World Congress of Peace Forces in Moscow, at a time when the Officials were becoming increasingly associated with Eastern Bloc socialism.

His popularity among the rank and file helped ease concerns within the movement, particularly for those who viewed the name change as a further departure from its republican roots.

He lost his Dáil seat at the general election later that year by a margin of just 59 votes to Liam Lawlor of Fianna Fáil.