The Emo Estate and other estates in Queen’s County were purchased in the early 18th century by Ephraim Dawson, a wealthy banker for whom Dawson Street in Dublin is named (Co. Laois). He married Anne Preston, the Emo Park Estate’s heiress, and settled in a home known as Dawson Court, which was near to the current Emo Court. In 1785, his grandson, John Dawson, was made 1st Earl of Portarlington. He married Lady Caroline Stuart, daughter of the Earl of Bute, who eventually became Prime Minister of England, three years later. In 1790, John Dawson hired Gandon to design Emo Court.

The 2nd Earl of Portarlington, also John Dawson, commissioned Lewis Vulliamy, a prominent London architect who had worked on the Dorchester Hotel in London, and A. & J. Williamson, Dublin architects, to finish the mansion when Gandon died in 1823, to be buried in Drumcondra churchyard. The dining room and garden front portico with huge Ionic columns were completed between 1824 and 1836, but the mansion remained unfinished when the 2nd Earl died in 1845. The double-height rotunda, drawing room, and library were not completed until 1860, when the 3rd Earl, Henry Ruben John Dawson, commissioned William Caldbeck, a Dublin architect, and Thomas Connolly, his contractor.