About 1829 he entered Trinity College, Dublin, but did not graduate. In 1830 Sheehan joined the Comet Club of young Irishmen, with Samuel Lover, Joseph Stirling Coyne, Robert Knox who became editor of The Morning Post, and Maurice O"Connell. The club issued pamphlets attacking the tithe system.
The first, The Parson"s Horn Book, which appeared in two parts with etchings by Lover, was popular.
Sheehan was appointed sub-editors In a few weeks it had reached a circulation of several thousand copies, and until its closure at the end of 1833 was influential.
The government in the autumn of 1833 ordered the arrest of Thomas Browne, the editor The Comet, and Sheehan, for libel. The fine was then remitted, and the term of imprisonment was only partly served.
Sheehan, on his release, studied for the Irish bar, to which he was called in 1835.
He shortly afterwards came to London. In 1836-1837 he was in Paris and Madrid as representative of The Constitutional newspaper. In 1839 Sheehan matriculated at Trinity College, Cambridge, though he took no degree.
He was admitted to the Inner Temple in 1841, and was called to the bar in 1846.
After a short career as a barrister he concentrated again on journalism. He was parliamentary reporter of the Morning Herald, contributing also poems and sketches to Bentley"s Miscellany and other magazines.
In 1852 Sheehan was proprietor and editor of The Independent of London and Cambridge. Subsequently in Temple Bar and elsewhere he often wrote under the signatures of "The Irish Whiskey-Drinker" and "The Knight of Innishowen".
Shortly after 1868 Sheehan married the widow of Colonel Shubrick, or Shrubrick, a wealthy Anglo-Indian officer, and spent some years in travelling on the continent.
He eventually retired to the London Charterhouse, where he died on 29 May 1882. William Makepeace Thackeray knew Sheehan well, and he is believed to be the original of Captain Shandon in Pendennis.