Irish international Cillian Sheridan was a prodigious Gaelic footballer in Cavan, and he also had another alternative to soccer: Aussie Rules. He, along with two other footballers, were approached by a representative from the Brisbane Lions: He began playing the association code with Bailieboro Celtic before travelling to Dublin to play for Belvedere, where he received call-ups for Irish youth squads. He was then offered a trial with Norwich before joining Celtic in 2006

He quickly progressed to the first team, making his Celtic dbut in the Scottish Cup quarter-finals against Inverness Caledonian Thistle on 25th February 2007, as a 73rd-minute substitute for Mark Wilson in a 2–1 away victory. On 20 May 2007, the final game of the season, he made his league debut against Hibernian, playing the final two minutes of a 2–1 defeat at Easter Road 

On 2nd February 2009, he was loaned to Motherwell. He made his debut five days later, coming on as a second-half substitute in a 1–1 draw with St Mirren in the Scottish Cup. He made his first start and league debut a week later on Valentine's Day in Motherwell's 1–0 win over Hamilton, playing the entire 90 minutes. Sheridan scored his first league goal for The Well a fortnight later, the winner in a 2–1 victory at Inverness

On 13 August 2009, Sheridan joined Plymouth on a six month loan deal.and played 14 games for Argyle.

On 14th January 2010, he returned to Celtic and was loaned out to St Johnstone, making his debut three days later against Hibs and scored two goals.

Sheridan signed a three-year contract with Bulgarian club CSKA Soia on 13th August 2010. He had played 19 games for Celtic.

In 2011, he trialled with American sides Seattle Sounders and Portland Timbers and with Scottish side, Hibernian before joining St Johnstone on loan in July 2011.

After being released by CSKA, he joined Kilmarnocj on 6th September 2012, making his debut against Hibs on 15th September.

On 20 June 2013, Sheridan signed a one-year contract with Cypriot side APOEl. He made his official debut on 17 August 2013 against Apollon Limassol in the Cypriot Super Cup final, in a match which APOEL won 1–0 and lifted the trophy. He helped them win the Cypriot league title and represented them in the Champions League. he won the domestic double again in May 2015.

On 10 June 2015, Sheridan signed a contract with rival Cypriot club Omonia..He made his debut on 9 July in the second leg of the Europa League first round qualifying tie against Dinamo Batumi

On 28 February 2017, Sheridan signed a two and half-year contract, with Polish side Jagiellonia Białystok

He was capped by Ireland at Under-21 level and won his first full cap on 25th May 2010 in a 2-1 win over Paraguay. He won two further caps.