| Viscountcy of Mountgarret | |
| Creation Date: | 1550 |
| Present Holder: | Piers James Richard Butler, 18th Viscount Mountgarret |
| Heir Apparent: | The Hon. Theo Oliver Stafford Butler (b. 2015) |
| Former Seat: | Nidd Hall, near Harrogate |
Viscount Mountgarret is a title in the Peerage of Ireland.
The title was created in 1550 for the Hon. Richard Butler, younger son of Piers Butler, 8th Earl of Ormond. Butler had largely rebuilt the tower house at Mountgarret in County Wexford.His grandson, the third Viscount, was outlawed and excepted from pardon in 1652, one year after his death. His son, the fourth Viscount, received a pardon for all treasons and rebellions from King Charles II and was restored to his estates.
He was succeeded by his son, the fifth Viscount who was a supporter of King James II and led the siege of Derry in 1688 to 1689. Lord Mountgarret was taken prisoner and outlawed, with his estates forfeited. However, in 1715 the outlawry was reversed and in 1721 he claimed his seat in the Irish House of Lords.
His great-grandson, the eleventh Viscount, represented County Kilkenny in the Irish House of Commons. He was succeeded by his son, the twelfth Viscount. He was made Earl of Kilkenny in the Peerage of Ireland in 1793. The earldom technically became extinct on his death in 1846, when he was succeeded in the viscountcy by his nephew, the thirteenth Viscount.
Despite the official title version, members of the Butler family have been known to refer to themselves as the rightful heirs to both the earldom and dukedom of Kilkenny. His son, the fourteenth Viscount, assumed in 1891 by Royal licence the surname of Rawson-Butler and the arms of Rawson (which were those of his maternal grandfather) when he inherited the Nidd Hall estate but in 1902 he resumed by Royal licence the surname of Butler only. In 1911 he was created Baron Mountgarret, of Nidd in the West Riding of the County of York, in the Peerage of the United Kingdom. the titles are held by his great-great-grandson, the eighteenth Viscount, who succeeded his father in 2004. He is understood to be the likely heir to the ancient earldom of Ormond (created in 1328) as well as to the 16th century earldom of Ossory, but has not successfully proven the claim.
The family seat is Stainley House, near Harrogate, North Yorkshire. The ancestral family home was Nidd Hall, also near Harrogate,
The heir apparent is the present holder's son, the Hon. Theo Oliver Stafford Butler (b. 2015).