His first wife was Margaret, eldest daughter of Hugh O"Neill, Earl of Tyrone, who was the most powerful Roman Catholic noble in the country at the time. His estates were nevertheless confirmed to him on the death of his father in 1605, and he sat in the parliaments of 1613, 1615, and 1634. By his first wife, Margaret, eldest daughter of Hugh O"Neill, Earl of Tyrone, he had three sons and six daughters, of whom.
He is best known for his participation in the Irish Confederate Wars on behalf of the Irish Confederate Catholics. Later, he was to clash politically with his grand-nephew James, the 12th Earl. He specially distinguished himself by his defence of the castles of Ballyragget and Cullihill.
At the commencement of hostilities in the Irish of 1641, he appeared inclined to espouse the Government side, and was appointed joint Governor of Kilkenny with the earl.
He then detached parties to secure other adjacent towns, which was done with such success that in the space of a week all the fortresses in the counties of Kilkenny, Waterford, and Tipperary were in their power. After this he was chosen general of the Catholic Confederation which the rebels had formed to coordinate their war effort.
But the county of Cork having insisted on choosing a general of its own. Thus were lost the advantages of undivided and vigorous control of the Confederate armies.
The Viscount"s forces were thereby considerably weakened, and he was defeated by the Earl of Ormonde at the Battle of Kilrush, near Athy, on 10 April 1642.
Returning to Kilkenny, he was chosen president of the Supreme Council (the Confederate Government) formed there in the following summer. In 1643 he was at the Battle of New Ross, fought by General Preston against the Marquis of Ormonde. He was with the Lords Netterville, Ikerrin, Upper Ossory, and Castlehaven at the siege of Ballinakill, which surrendered on 5 May.
Mountgarret was outlawed by Cromwell, and excepted from pardon for life or estate.
The Viscount died in 1651, and was interred in Saint Canice"s, Kilkenny. Although he was dead before it passed, he was excepted from pardon for life or estate by the crown in the Acting of Settlement 1652 passed on 12 August
1652, because of his participation in the rebellion of 1641.
Margaret married another member of the Butler line, Sir Walter Butler, 1st Baronet Polestown.
