Seán O'Μahony (also John O'Mahoney) (20 October 1872 – 28 November 1934) was an Irish Sinn Féin politician and member of the First and Second Dáil.
He was born as John Mahony in Logan Street, Thomastown, County Kilkenny, to James Mahony, a tailor, and Mary Cantwell. A successful businessman he was a tea merchant and a commercial traveller. His company, John O'Mahoney & Co., was located on Middle Abbey St., and was destroyed during the 1916 Easter Rising. It subsequently reestablished on Parnell Square. He subsequently purchased and ran Fleming's Hotel which was located at 31-32 Gardiner Place, Dublin. A close friend of Arthur Griffith, he became an organiser for Sinn Féin and was elected to Dublin Corporation for the party.
He participated in the Easter Rising and was subsequently interned at Frongoch and Lincoln Jail. He remained with Sinn Féin after it was re-constituted as a republican party at the 1917 Ardfheis.
He was arrested during the German Plot of 1917 and was jailed in Lincoln Gaol in England. While imprisoned he was elected as a Sinn Féin MP for Fermanagh South in the 1918 general election. He was released from prison in 1919 and attended the proceedings of the First Dáil. He was re-arrested at the Sinn Féin offices in November 1919 and was imprisoned for three months in England. He would be arrested several more times throughout the Anglo-Irish War and his hotel was used as a meeting place by Sinn Féin members throughout the time.