He was born on 18 July 1978 in Bellewstown, County Meath, to an Irish mother, Ursula (née Campbell) from County Kildare whose parents before her originated from Midfield near the towns of Kilkelly Kiltimagh and Swinford in County Mayo and a New Zealand father, John. When Horgan was young he played for Boyne RFC. He also played Gaelic football at Minor (U18) level with Meath. He was educated at a boys' Catholic school, in Drogheda, and was active in their rugby team. Horgan joined Lansdowne on leaving school in 1997.
Horgan made his debut for Leinster in 1998. He played for Leinster 87 times in the Heineken Cup, ranking fifth in career Heineken Cup appearances. He scored 27 tries during his Heineken Cup career, ranking him fourth on the list of most career tries scored in Heineken Cup competition. His most prolific Heineken Cup season was 2004–05, when Horgan was the leading try scorer with 8 tries. He was a member of a Leinster backline which included Brian O'Driscoll, Gordon D'Arcy, Denis Hickie, Felipe Contepomi, Isa Nacewa, Rob Kearney and Jonathan Sexton.
Horgan made his debut for the Ireland senior side against Scotland in the 2000 Six Nations Championship, scoring a try in the process. In the following game against Italy he scored 2 tries and added another one against Wales during the tournament. He was selected for the 2005 British and Irish Lions tour to New Zealand. He was used as a substitute during all three tests. After scoring the winning try against England at Twickenham in the 2006 Six Nations Championship to secure the Triple Crown for the second time in three years, he became a new national hero. Horgan added to his following in 2007 when he scored a try against England in Croke Park by executing a Gaelic football style catch from a Ronan O'Gara crossfield kick. Horgan is known by the Irish media and rugby fans as "Shaggy".
Horgan showed a return to something near the levels of performance of his earlier career in early 2011 but injury towards the end of the club season forced him to miss out on the 2011 Rugby World Cup.
Horgan has three older sisters Maria, Sharon and Lorraine and one younger brother, Mark.
He is a second cousin of Jockey Leighton Aspell and his brother Paddy.