James Clarence Mangan, born James Mangan (Irish: Séamus Ó Mangáin; 1 May 1803 – 20 June 1849), was an Irish poet. He freely translated works from German, Turkish, Persian, Arabic, and Irish, with his translations of Goethe gaining special interest. Starting around 1840, and with increasing frequency after the Great Famine began, he wrote patriotic poems, such as A Vision of Connaught in the Thirteenth Century. Mangan was troubled, eccentric, and an alcoholic. He died early from cholera, amid the continuing dire conditions of the Famine. After his death, Mangan was hailed as Ireland's first national poet and admired by writers such as James Joyce and William Butler Yeats.

Mangan was born at Fishamble Street, Dublin, the son of James Mangan, a former hedge school teacher and native of Shanagolden, County Limerick, and Catherine Smith from Kiltale, County Meath. After marrying Smith, James Mangan took over a grocery business in Dublin owned by the Smith family, eventually becoming bankrupt as a result.

Mangan described his father as having "a princely soul but no prudence", and attributed his family's bankruptcy to his father's suspect business speculations and tendency to throw expensive parties. Thanks to poor record keeping, inconsistent biographies, and his own semi-fictional and sensationalized autobiographical accounts, Mangan's early years are the subject of much speculation. However, despite the popular image of Mangan as a long-suffering, poor poet, there is reason to believe that his early years were spent in relative middle-class comfort.

He was educated at a Jesuit school where he learned Latin, Spanish, French, and Italian. He attended three schools before the age of fifteen. Obliged to find a job in order to support his family, for seven years he was a scrivener's clerk and for three years earned meagre wages in an attorney's office, and was later an employee of the Ordnance Survey and an assistant in the library of Trinity College, Dublin.^{[citation needed]}

Mangan's first verses were published in 1818. From 1820 he adopted the middle name Clarence. In 1830 he began producing translations – generally free interpretations rather than strict transliterations – from German, a language he had taught himself. Of interest are his translations of Goethe. From 1834 his contributions began appearing in the Dublin University Magazine. In 1840 he began producing translations from Turkish poetry, Persian poetry, Arabic poetry, and Irish poetry. He was also known for literary hoaxes; some of his "translations" are in fact works of his own, like Twenty Golden Years Ago, attributed to a certain Selber. His connection with The Dublin University Magazine was terminated because his habits rendered him incapable of regular application.

He was friends with the patriotic journalists Thomas Davis, and John Mitchel, who would write his biography. His poems were published in their newspaper The Nation. Mangan was for a time paid a fixed salary, but, as on former occasions, these relations were broken off, though he continued to send verses to "The Nation" even after he had cast in his lot with Mitchel, who in 1848 began to issue The United Irishman.

Although his early poetry was often apolitical, after the Famine he began writing patriotic poems, including influential works such as Dark Rosaleen, a translation of Róisín Dubh and A Vision of Connaught in the Thirteenth Century.

His best-known poems include Dark Rosaleen, Siberia, Nameless One, A Vision of Connaught in the Thirteenth Century, The Funerals, To the Ruins of Donegal Castle, Pleasant Prospects for the Land-eaters and Woman of Three Cows. He wrote a brief autobiography, on the advice of his friend Charles Patrick Meehan, which ends mid-sentence. This must have been written in the last months of his life, since he mentions his narrative poem of the Italian Gasparo Bandollo, which was published in the Dublin University Magazine in May 1849.