A prolific and uneven writer of almost every kind of verse whose best work, inspired by love of Ireland, ranks high in Irish poetry.
The son of an unsuccessful grocer, at the age of 15 he became a copying clerk in a scrivener´s office and remained one for 10 years after which he lived as best he could contributing to the prestigious Dublin University Magazine and other literary periodicals though posts were found for him for brief periods in the library of Trinity College, Dublin, and the Ordnance Survey Office.
His natural melancholy was aggravated by years of ill-paid drudgery and an acute disappointment in love. He became an opium addict and a chronic drunkard and the last years of his life were spent in extreme neglect and wretchedness. When he died of cholera only two persons attedended his funeral.
Many of his poems are "translations" from the Irish, fromm German and from various Eastern languages (which Mangan probably did not know) often so free that Mangan is in effect using the original as a vehicle for his own emotions. He often also described as translations poems that were, in fact, altogether his own.
Much of his work has Irish history and legend for its theme but in a few poems such as The Nameless One and Twenty Golden Years Ago he achieves an extraordinary modern note of personal realism and a tragic sincerity of tone. Perhaps his best known poem is Dark Rosaleen in which, in the characteristic Irish fashion, Ireland is a woman whom he adresses as his beloved.