Club Shanahan played his club hurling with the
Lismore club in Waterford. He had several successes at underage levels winning county medals in hurling at under-14, under-16, minor and under-21 levels. Shanahan was only sixteen years-old when he won a senior
county club championship in 1993 following a one-point defeat of
Passage. The following years proved to be somewhat frustrating for Shanahan and Lismore. Ballygunner and Mount Sion both had very strong teams and dominated the county scene for a number of years. Despite having a depth of talent within their ranks, Lismore were unable to win another county title in this period, going very close on a number of occasions most notably in the 1996, 2001 and 2009 county finals. In 2016 and at 39 years old, Shanahan finally captured another county medal, this time in the intermediate grade. This win was followed by Lismore winning the Munster Intermediate championship with a victory over Kerry side Kilmoyley. He was still playing for Lismore at the age of 45 in the
2022 championship. He had announced his retirement at the conclusion of the season, playing his last game for Lismore against
Mount Sion in the Waterford SHC quarter-final. He returned to play for Lismore in 2023 and in the following year he came on as a substitute in the
2024 championship relegation play-off win on 16 August against
Tallow, it was his 32nd consecutive season playing adult club hurling.
Inter-county Shanahan's inter-county career began with the Waterford minor and under-21 teams. He was a member of the Waterford team that reached the Munster minor final in 1994. However, Cork ran out comfortable winners. The following year Shanahan and Waterford were back in the Munster final against the same opposition, and once again it was the Cork men who emerged victorious. He was also a member of the Lismore CBS side that reached the Harty cup final the same year. A star studded Midleton CBS outfit were the opposition, and despite Shanahan being amongst the goals, it was the east Cork school who took home the cup. He joined the Waterford senior team in 1995, however, he did not make his senior championship debut until 1998. That year Waterford held All-Ireland champions
Clare to a draw in the Munster final, however, the Decies lost the replay. Shanahan's side later faced
Kilkenny in the All-Ireland semi-final in what would be the county's first appearance in
Croke Park since
1963. Kilkenny won by a single point. In 2002 Waterford emerged from the doldrums by reaching the Munster final once again, however, Shanahan could not command a definite place on the starting fifteen. In spite of this he came on as a substitute to claim his first
Munster winners' medal as Waterford defeated Tipperary by 2–23 to 3–12. It was their first provincial crown in thirty-nine years. After surrendering the Munster title to Cork in 2003, Shanahan's side were back in the provincial showpiece for a third successive year in 2004. He was by now a regular on the starting fifteen. In the Munster final, Waterford defeated Cork for the first time in forty-five years to take the title by 3–16 to 1-21. An All-Ireland appearance subsequently eluded the team yet again. In spite of this there was some consolation at the end of the year as Shanahan's scoring prowess earned him his first
All-Star award. The following few years proved difficult for Shanahan. Waterford crashed out of the early stages of the provincial championship in 2005 and 2006. On both these occasions his team was defeated by Cork in the All-Ireland series. He did, however, win a second All-Star award in 2006. In 2007 Shanahan added a
National Hurling League medal to his collection when Waterford defeated Kilkenny by 0–20 to 0–18 in the final. He later claimed a third Munster winners' medal as Waterford defeated
Limerick by 3–17 to 1–14 in the provincial decider. Shanahan himself scored a hat-trick of goals in the second half. While Waterford were viewed as possibly going on and winning the All-Ireland title for the first time in almost half a century, Limerick ambushed Waterford in the All-Ireland semi-final. In spite of falling short of championship success Shanahan's tally of eight goals over the whole championship marked him out as one of the best forwards in the country. He was later presented with a third All-Star award as well as claiming the
Vodafone Hurler of the Year award. Shanahan later made a clean sweep of all the player of the year awards by also collecting the
GPA Hurler of the Year and
Texaco Hurler of the Year awards as well. 2008 began with Waterford losing their opening game to
Clare as well as their manager
Justin McCarthy. In spite of this poor start Shanahan's side reached the All-Ireland final for the first time in forty-five years. Kilkenny provided the opposition and went on to defeat Waterford by 3–30 to 1–13 to claim a third All-Ireland title in-a-row. Shanahan was relegated to the subs' bench in 2010 as Waterford faced Cork in another Munster final. A 2-15 apiece draw was the result on that occasion, however, Waterford went on to win the replay after an extra-time goal by Shanahan. It was a fourth Munster winners' medal for Shanahan, a record that he shares with five other Waterford players. In August 2010 he announced his retirement from the inter-county game after Waterford's exit to
Tipperary in the
All Ireland semi final.
Inter-provincial Shanahan was also a veteran of five
inter-provincial championship campaigns with
Munster. He first lined out with his province in 1998, however, he did not capture a winners medal in this competition until 2007 when Munster defeated
Connacht by 2–22 to 2–19. ==Post-playing career==