Coupe d'Europe de rugby à XV
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.