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.
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.
[3] 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.
Mangan was a lonely and often difficult man who suffered from mood swings, depression and irrational fears, and became a heavy drinker and opium user.
There are many descriptions of his personal appearance at this time, all of them dwelling on his spare figure, his tight blue cloak, his witch's hat, and his inevitable umbrella.
He was described by the artist William Frederick Wakeman as frequently wearing "a huge pair of green spectacles", padded shirts to hide his malnourished figure, and a hat which "resembled those which broomstick-riding witches are usually represented with".
Most obviously, and frequently, his work is read alongside such nationalist political authors as John Mitchel, as they appeared in The Nation, The Vindicator and the United Irishman newspapers; or as a manifestation of the 19th-century Irish Cultural Revival.
In particular, he is compared to Samuel Taylor Coleridge and Thomas De Quincey, largely thanks to his rumoured opium addiction and tendency to place his writing within the frame of a vision or dream.
Private papers of Mangan are held in the National Library of Ireland, the Royal Irish Academy, and the archives of Trinity College, Dublin.