Byrne is one of the two sons of Colclough Byrne of Drumquin, Hackettstown and Mary Galway of Cork, a great grand-niece of James Butler, 1st Duke of Ormond. He lives most of his adult life at Park Hill in the Glen of the Downs, County Wicklow. In late 1796, he enlists in the yeomanry, serving in the Newtown Mount Kennedy cavalry.
Byrne joins the Society of the United Irishmen in the spring of 1797 and later that same year is appointed by the Leinster committee to organise the half barony of Rathdown. As a delegate for Rathdown barony, he is a well respected and competent figure. With the assistance of his Protestant friend Thomas Miller of Powerscourt, he undertakes the organisation of military and civil branches of the United Irishmen in Rathdown, recruiting 2,000 men by late 1797. In October 1797 he is forced to resign from the yeomen after refusing to swear the oath of loyalty and his activities begin to come to the attention of Dublin Castle.
According to the informant A.B. (Thomas Murray), Byrne attends the inaugural meeting of the United Irishmen’s Wicklow county committee in December. It is held in the Annacurra home of William’s first cousin, John Loftus. Murray’s information tells that Byrne established networks of contacts between the Leinster committee and the Cork United Irishmen along with other contacts in Munster.
Byrne’s career comes to an end when he, along with fourteen other Leinster delegates, are arrested on March 12, 1798 at the house of Oliver Bond. They had been betrayed by Thomas Reynolds, treasurer of Kildare United Irishmen and member of the provincial committee. Reynolds had been informed that plans for an insurrection were about to be finalised by the committee. Byrne is arrested in possession of incriminating documents which are described by Attorney-General for Ireland Arthur Wolfe as being ‘very treasonable printed papers.’
On July 4, 1798 Byrne with four others is brought before a commission of oyer and terminer on charges of high treason. The case mounted by the state against him is based principally on the evidence of Reynolds and also that given by his former comrade William Miller of Powerscourt. The weight of the evidence is overwhelming, rendering an effective defence impossible.
Byrne’s lawyer, John Philpot Curran KC, the leading defence counsel of the period, attempts to cast Reynold’s character and motives in a foul light but it is futile. In his last days, efforts are made to spare his life if he would only express regret for his actions and accuse Lord Edward FitzGerald for having led him to this point. He refuses, meeting his end with great dignity and stoicism.
Byrne is convicted of high treason and executed on July 25, 1798 outside Green Street Courthouse, Dublin.
The Dublin Magazine notes that Byrne “met his fate with a degree of courage perhaps unequalled.” For his service to the state, Reynolds is honoured by Dublin Corporation with the freedom of the city on October 19, 1798, spending much of his life thereafter in fear of assassination.