The house was designed by the architect Richard Cassels with later additions by Thomas Ivory and James Wyatt.

The title and the house were separated in 2014, following the death of Jeremy Browne, 11th Marquess of Sligo, who left the estate to his five daughters.

His titles passed to his first cousin, Sebastian Ulick Browne, a residential estate agent in Australia.

Colonel John Browne (1638–1711), built the first Westport House on the site of the O'Malley castle of Cahernamart.

[1] The house was rebuilt by the Browne family in the 18th century to a design by the architect Richard Cassels in a Palladian style.

James Wyatt later redesigned one of the interior dining rooms in 1781 while later in 1796 he designed a greenhouse for John Browne, 3rd Earl of Altamont.

After the fire, the Second Marquess covered in the open courtyard and made a new library by running a gallery around the now enclosed interior wall.

In 2007, the privately owned estate received a grant of €1.314m for repairs to Westport House, from the state funded Heritage Council.

Denis Browne, brother of the 1st Marquess of Sligo and a member of Grattan's Parliament; and by William Beechey, of Howe Peter – the 2nd Marquess of Sligo, who spent four months in an English jail for bribing British seamen in time of war to bring his ship, full of antiquities from Greece, to Westport.

There is a portrait of Earl Howe – Admiral of the Fleet, father of the 1st Marchioness of Sligo, by John Singleton Copley.

Maud Bourke, daughter of The 3rd Viscount Mayo and great-great-granddaughter of 'the Pirate Queen', Granuaile (Grace O'Malley; 1530–1603), in 1669.

[9] Browne greatly increased his estate in Mayo and Galway including Cahernamart (Cathair-na-Mart - the Fort of the Beeves), a ruinous O'Malley fortress on the shores of Clew Bay.

The Penal Laws which followed left his grandson, John IV, with little option but to conform to the prevailing religion in the hope of surviving the confiscations and political upheaval.

Young and ambitious, he set about extending his estate and transforming the old O'Malley castle into modern-day Westport House.

In 1752, his son and heir, Peter, 2nd Earl of Altamont, married the heiress, Elizabeth Kelly from County Galway, whose estates in Jamaica further enhanced the family fortune.

He created the lake to the west of Westport House, planted trees and employed James Wyatt to decorate the gallery and dining room.

Aided by the arbitrary actions of Denis Browne, his younger brother, against the Irish insurgents (which earned him the reputation of 'black sheep' of the family), the Rebellion was crushed.

To bring them back to Westport, he took some seamen from a British warship and was subsequently sentenced to four months in Newgate Prison.

He married Lady Hester de Burgh, the Earl of Clanricarde's daughter, by whom he had fourteen children, and settled down to life in Westport.

The 2nd Lord Sligo added the north and south wings, the library, and commissioned much furniture, china, silver and paintings for the House.

In 1834, he was appointed Governor of Jamaica with the difficult task of overseeing the 'apprenticeship system', in a period prior to the full emancipation of the slaves.

Westport House was closed and with no rents forthcoming, the 3rd Lord Sligo borrowed where he could, spending £50,000 of his own money to alleviate the suffering of the tenants.

[1] He had to contend with the huge changes that occurred in the ownership of land in Ireland in the late nineteenth and early twentieth century.

An active man, he rebuilt the stable block and made the terraces above the lake on the west side of the house..

HE installed central heating, electric light and six bathrooms, and ensured the house was in a good state of repair.

[16] He spent the latter part of his life fighting against slavery both as Governor General of Jamaica and afterwards in the House of Lords.

The Jamaican plantations ('Kellys pen' and 'Cocoa Walk') and slave workers, originally came into Browne family ownership through the marriage in 1752 of Peter Browne to Elizabeth Kelly, only daughter and heir of Dennis Kelly, from County Galway who became Chief Justice of Jamaica in the 18th century.

[17] Some of the acts that performed were Jools Holland, The Waterboys, Ryan Sheridan, Imelda May, Seasick Steve, Mundy, and Royseven.

[18] On 26–27 August 2017, Aiken Promotions used Westport House as one of two venues for the first Harvest Country Music Festival.