Michael was educated at Dr Magrath's Catholic school. He went on to study for the bar, but a decline in his father's business caused him to retire from his studies. He returned home to take over the family business, which he returned to prosperity. He married in 1840. He was appointed postmaster of Kilkenny in 1852 which he held until illness forced him to retire in 1873. He died in Dublin in 1873. His widow was granted a civil list pension.
He assisted John in the O'Hara Tales, where he used the name "Abel O'Hara," and there is difficulty in allocating their respective contributions. While John was the more experienced writer, Michael provided material based on his social observations. They revised each other's work; according to Patrick Joseph Murray's Life of John Banim. Michael was the principal author of about thirteenout of the twenty-four works attributed to the brothers including Crohoore of the Bill-Hook, The Croppy, and Father Connell. After the death of John, Michael wrote Clough Fionn (1852), and The Town of the Cascades (1864). In 1861 he wrote prefaces and notes for a reprint of the "O'Hara" novels by the Catholic publishing firm Sadleir of New York.