In London with Margaret, who was visiting her parents, Gannon went to the Benton Fletcher collection of keyboard instruments, which was then in Chelsea, and measured a harpsichord by Jacob and Abraham Kirckman (1777). Back home, he made a copy of the instrument in a tiny conservatory at the back of his house in the Dublin suburb of Rialto. The harpsichord was played by John S. Beckett for the first time in public in 1959 as the continuo for Bach's Saint Matthew Passion and was praised in the national press. Beckett subsequently persuaded the authorities in the Guinness Brewery to provide Gannon with a special workshop, in which he made five harpsichords and restored several antique pianos. The first harpsichord made in the Brewery was donated to the Royal Irish Academy of Music in Dublin, The second was sold to Harrods of London, and the third was sold to Ireland's national radio and television station RTÉ. This third instrument was used regularly by the RTÉ Symphony and Concert orchestras and also by the well-known composer and performer of Irish traditional music, Seán Ó Riada.