McHugh began refereeing in the 1980s. He would later recall taking charge of a schoolboy international between a Wales team featuring Scott Quinnell and an England team captained by Kyran Bracken. McHugh made his senior international debut as a referee on 17 September 1994 when he took charge of a 1995 Rugby World Cup qualifier between Romania and Wales at Dinamo Stadium.
McHugh refereed matches at the 1995, 1999 and 2003 Rugby World Cups. He made his Rugby World Cup debut on 3 June 1995 taking charge of a match between Canada and South Africa. McHugh issued three red cards, sending off Canada's Rod Snow and Gareth Rees and South Africa's James Dalton following a brawl. McHugh went onto referee six Rugby World Cup matches over three tournaments.
Tri Nations assault
On 10 August 2002 McHugh was refereeing a Tri Nations match between South Africa and New Zealand at the ABSA Stadium. McHugh had awarded New Zealand a penalty try, after a high tackle on Tana Umaga, and denied South Africa's Breyton Paulse a try after an interference call against James Dalton. These incidents allegedly incited Pieter Van Zyl, a South African supporter, to invade the pitch and tackle and assault McHugh. Van Zyl evaded over four hundred security officials to launch the assault as New Zealand'sJustin Marshall was preparing to put the ball into a scrum. New Zealand's Richie McCaw punched Van Zyl and, together with South Africa's AJ Venter, wrestled the assailant off McHugh. Van Zyl was then escorted off the pitch by two security officials. The assault left McHugh with a dislocated shoulder and he had to be replaced as the match referee by Chris White. Van Zyl was subsequently found guilty of assaulting McHugh and fined R10,000. He also received a lifelong ban, preventing him from attending matches organised by the South African Rugby Union.
After retiring as a referee, McHugh continued to be employed by the IRFU in various roles. He served as specialist advisor on the 2005 British and Irish Lions tour to New Zealand. He also acted as a television match official and as an assessor. However his main role was to work as a referee development officer. McHugh would help recruit and mentor both John Lacey and Joy Neville. In August 2015 McHugh became the IRFU referee performance manager. This involved coaching and managing the IRFU's elite panel of referees who took charge of matches in the Pro14, the European Rugby Champions Cup and the European Rugby Challenge Cup as well as test matches. On 5 October 2017 his contract with the IRFU was terminated, following an independent review of the referee department. McHugh subsequently took legal action, involving the High Court, against the IRFU, claiming unfair dismissal. However on 7 November 2017 it was announced that the McHugh and the IRFU had reached a settlement.
