Barret's first major patron was probably Joseph Leeson, 1st Earl of Milltown, who built the Palladian mansion, Russborough House in the southern part of County Wicklow.Richard Cassels was the architect and the house was furnished by Joseph Leeson, who travelled on grand tours of Europe in 1744 and again in 1750, amassing a large art collection of paintings, sculpture, furniture, and antiques. This was augmented by classical landscapes and local landscapes. Barret's paintings were used as a decorative scheme in the newly created dining room Many of Barret's paintings passed with the Milltown bequest to the National Gallery of Ireland in 1902.