Between 1981 and 1988, MacNeill made 37 appearances for Ireland. He made his debut for Ireland on 7 February 1981 against France at Lansdowne Road, scoring the first of ten international tries. In the same championship he scored a second try against Wales and a drop goal against England. He scored further tries against England in 1982 and Scotland in 1988. He also scored a penalty against Wales in 1983. MacNeill was a member of the Ireland teams that won the Five Nations Championship and Triple Crown in 1982 and 1985 and shared the championship with France in 1983. He was also a member of the Ireland squad at the 1987 Rugby World Cup, where he scored four tries in Ireland's four games, one against Canada, two against Tonga and one in the quarter final defeat to Australia. MacNeill made his final appearance for Ireland against England on 23 April 1988. Together with Tony Ward, Moss Keane, Donal Spring and Ciaran Fitzgerald, MacNeill declined to take part in the 1981 Ireland rugby union tour of South Africa during the apartheid era. In the aftermath of the 1996 Docklands bombing, together with Trevor Ringland, MacNeill helped organise a friendly between Ireland and the Barbarians at Lansdowne Road to show that the people of Ireland wanted peace.