MacNeill completed his secondary level education at Blackrock College. On 20 March 1977 he was a member of the Blackrock College team that won the Leinster Schools Rugby Senior Cup, defeating St Mary's College 24–12 after extra time at Lansdowne Road. The St Mary's team featured MacNeill's future Ireland teammate Paul Dean. MacNeill scored a conversion and two penalties in extra time to help seal victory for Blackrock.
MacNeill subsequently attended Trinity College Dublin where he earned a degree in economics and a diploma in Anglo-Irish literature. He was elected a Scholar of the College in 1979. His lecturers included Brendan Kennelly and David Norris. While at Trinity he played for both the rugby union and association football teams. On 26 November 1980 he captained Dublin University to a 9–3 win over University College Dublin in The Colours Match. He also went on a rugby union tour to Japan and represented the team in games against both Oxford University and Cambridge University. He played association football for both University College Dublin and Dublin University A.F.C.. In the 1978–79 League of Ireland B Division season he was the top scorer for UCD with 46 goals. In 1979 he was a member of the Dublin University team, coached by Liam Tuohy, that won the Collingwood Cup. MacNeill scored twice in the final as Dublin University defeated Maynooth University 2–0.
Oxford University
MacNeill also studied economics at St Edmund Hall, Oxford. Between 1982 and 1984 he played in three Varsity matches for Oxford University against Cambridge University but lost all three. He captained the Oxford team in 1983.
International career
Ireland
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.
British and Irish Lions
MacNeill made three test appearances for the British and Irish Lions during their 1983 tour of New Zealand. MacNeill scored a penalty against Auckland in the first of his nine appearances on the tour. He started the first two matches against the All Blacks before coming off the bench to replace Ollie Campbell in the final test.
Barbarians
On 22 October1989 MacNeill played for the French Barbarians against Fiji. Fiji won the game 32–16.
Later years
Between 1982 and 2000 MacNeill worked in London, initially for the Boston Consulting Group and later for Goldman Sachs. In 2000 he returned to Dublin where he continued to work for Goldman Sachs as a director of their investment branch in Ireland. He has also served as chairman of several charities including GOAL, The Ireland Funds and the British Irish Association. In 2010 MacNeill married Dr. Jennifer Carroll. Carroll is a UCD graduate, barrister and author who has served as a legal adviser to both Fine Gael and the Government of Ireland. Among those she has advised are Enda Kenny, Alan Shatter and Frances Fitzgerald.. She was elected as a Teachta Dála for the Dún Laoghaire constituency at the 2020 general election. In 2014 MacNeill was appointed chairman of a working group established to help bring the 2023 Rugby World Cup to Ireland. He also worked as a pundit for TV3 during their 2015 Rugby World Cup coverage. He has also worked as a pundit for BBC Radio 5 Live during Six Nations Championships. In February 2020 MacNeill retired from Goldman Sachs to stand for Seanad Éireann in the Dublin University constituency.
Honours
Rugby union
;Ireland
Five Nations Championship/Triple Crown : 2
* 1982, 1985
Five Nations Championship: 1
* 1983 '
;Dublin University
The Colours Match: 1
* 1980
;Blackrock College
Leinster Schools Rugby Senior Cup: 1 '''
* 1977
;Notes
Ireland and France shared the championship in 1983.
