He was a successful lawyer in late 18th and early 19th century Dublin, and wrote a law book that was crucial in the development of the "beyond reasonable doubt" standard in criminal trials.

However, during his time, he was best known for his popular comic operas and plays, together with his most enduring work, the romantic song "The Lass of Richmond Hill".

In return for payments from the government, McNally would betray his United Irishmen colleagues to the authorities and then, as defence counsel at their trial, secretly collaborate with the prosecution to secure a conviction.

His notable republican clients included Napper Tandy, Wolfe Tone, Robert Emmet and Lord Edward FitzGerald.

[6] In the early 1790s, McNally became a founder member of the United Irishmen,[9] a clandestine society which soon developed into a revolutionary Irish republican organisation.

[14] After his death in 1820, it emerged that McNally had for many years been an informant for the government, and one of the most successful British spies in Irish republican circles that there has ever been.

[13] When, in 1794, a United Irishmen plot to seek aid from Revolutionary France was uncovered by the British government, McNally turned informer to save himself,[13] although, subsequently, he also received payment for his services.

[17] During the trial of Emmet, McNally provided details of the defence's strategy to the crown and conducted his client's case in a way that would assist the prosecution.

[8] After McNally's death, his activities as a government agent became generally known when his heir attempted to continue to collect his pension of £300 per year.

[21] One of his songs, Sweet Lass of Richmond Hill, became very well-known and popular following its first public performance at Vauxhall Gardens in London in 1789.