Seán O'Mahony (also John O'Mahoney)[1] (20 October 1872 – 28 November 1934) was an Irish Sinn Féin politician and member of the First and Second Dáil.

A close friend of Arthur Griffith, he became an organiser for Sinn Féin and was elected to Dublin Corporation for the party.

De Valera was keen for him to attend since if he had been refused entry it would have demonstrated, in the minds of Anti-Treaty supporters, that the assembly was not an All-Ireland Dáil.

[7] This was to demonstrate de jure existence of the Second Dáil as a representative for Fermanagh was unable to attend the assembly, the point made by Ginnell and argued in theoretical republican theology.

O'Mahony remained an abstentionist MP to Stormont until the 1925 Northern Ireland general election, which he did not contest.

Remaining with Sinn Féin after the 1926 split he served on the party's Ard Chomairle until his death.