Sustaining faith in the idea of an Irish Republic was a troubling challenge after the end of the Civil War in 1923. Many defeated anti-Treaty IRA members faced a bleak vista, and up to 600 of them had emigrated by 1924, referred to as a “lost legion” by the writer Frank O’Connor. His fellow writer Seán O’Faoláin elaborated meanly on their dejection: “these febrile, fractious, bitter, hungry-eyed ex freedom fighters were now in every sense out of a job”.

Even during the Civil War, Éamon de Valera struggled with the faith; he told Cork republican Mary MacSwiney in 1922: “reason rather than faith has been my master ... I have felt for some time that this doctrine of mine ill fitted me to be leader of the republican party”.

That description – “republican party” – was the preferred option of Seán Lemass for the new party, Fianna Fáil, which emerged a century ago, after de Valera resigned the presidency of Sinn Féin to remove himself from what he perceived as a political cul de sac. De Valera preferred “Fianna Fáil”; it had the advantage, he later recalled, of being difficult to translate; exactitude was not in his interests at that stage. Evoking the legend of Fionn Mac Cumhaill was preferred, with the added appeal that the initials FF (“the legendary first standing army of........

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