Following his education MacEntee worked as an engineer in Dundalk, County Louth, and was involved in the establishment of a local corps of the Irish Volunteers in the town.

MacEntee was released in the general amnesty in 1917 and was later elected a member of the National Executives of both Sinn Féin and the Irish Volunteers in October 1917.

He was also a member of the Volunteer Executive, a sort of Cabinet and Directory for the Minister for Defence and the HQ Staff[7] But MacEntee remained one of the few Sinn Féiners from the north.

Sean MacEntee was asked to resign his South Monaghan seat, after voting against a bunting celebration in Lurgan to mark the signing of the Anglo-Irish Treaty.

[citation needed] From April 1921, when MacEntee was transferred to Dublin to direct a special anti-partition campaign in connection with the May general election.

It remained Collins's policy, he declared, that the largely Protestant shipyard workers of Belfast were being directed by the British, urging all Irishmen to rejoin the Republic.

[11] Correspondingly the Ulster Unionist Council rejected the call for a review of the boundary commission decision made on Northern Ireland.

[13] During the subsequent Civil War MacEntee commanded the IRA unit in Marlboro Street Post Office in Dublin.

In keeping with the party's protectionist economic policies, his first budget in March of that year saw the introduction of new duties on forty-three imports, many of them coming from Britain.

This was the beginning of the Economic War between the two nations, however, a treaty in 1938, signed by MacEntee and other senior members brought an end to the issue.

[16] In 1939, World War II broke out and a cabinet reshuffle resulted in MacEntee being appointed as Minister for Industry and Commerce, taking over from his rival Seán Lemass.

The financial and economic portfolios were dominated by Lemass and other like-minded ministers who wanted to move away from protection to free trade.

[19][20][21][22] In June 1921, he married a strongly nationalistic woman from County Tipperary, Margaret Browne (1893–1976), who later taught Irish at Alexandra College and then at UCD.

[26][27][28] Barbara and Frank's third son was Maurice Biggar (1956–2023), a "young star of L&H debates, diplomat, barrister, gaeilgeoir, linguist and poet".