Michael, the son of Andrew and Mary Cullinane, was a native of Cappoquin. He had seven sisters. The effects of the Famine and the evictions of tenants of Arthur Keily Ussher are said to have influenced him in joining the Young Ireland movement. He wrote his 'Memoirs ' of  Thomas Francis Meagher, whom he first met in Dublin in 1848 with Thomas Devine Reilly and John Mitchel. He blamed the failure of the movement on the lack of military knowledge among the leaders and the absence of preliminary organisation. During 1848 Cavanagh met and became life time friends with John O'Mahony, a member of the movement in Carrick. Cavanagh, Hugh Collender and Dan McGrath (local Members) left for America in 1849, when a reward of £100 each was offered for their capture. He joined the Irish Republican Union. He practised the family trade of cooper in Shushan, Washington County in upstate New York. He later moved to Waterford, Saratoga where he was granted American citizenship in 1855.

By 1863, at the time of his marriage to Anne O'Brien, Affane, his address was given as 6 Centre Street, New York, the Fenian headquarters. Cavanagh began writing for the Emerald, in 1868, a literary illustrated weekly published in New York. He also contributed to the Irish People, an O'Mahony publication. He went to Washington in 1870 and enlisted in the United States Army as a Civil Servant, which he held until his death. He wrote many published poems and articles during this period. Cavanagh died on 21 June 1900 and was buried in Mount Olivet Cemetery, New York. A bronze bust was recently erected in the square in Cappoquin to Cavanagh as part of the Cavanagh weekend.