He studied at Queen's University, Belfast and was called to the bar in 1979.
Two decades later, in 1999, he was appointed Queen's Counsel, and the following year was called to the Bar Council of Ireland.
[1][2] Along with Barry MacDonald, Treacy challenged the ruling that barristers must swear allegiance to the Queen, arguing successfully that lawyers who held nationalist views should be exempt from this requirement.
[3] In 2009, following the discovery of a pipe bomb nearby, Treacy moved from his Belfast home due to security concerns.
[3] In 2018, he resigned from the Privy Council, only a few months after his appointment.