Crystal Palace Scannell joined Palace at 14 and notched 23 goals for the academy during the 2006–07 season, graduating to the reserves before breaking into the first team in the 2007–08 season. Scannell made his debut for Crystal Palace in a 2–1 win at London rivals QPR in December 2007 as a substitute. In his home debut against
Sheffield Wednesday on 15 December, he again came on as a substitute in the second half and scored a 90th-minute winner, his first senior goal in a 2–1 victory for Crystal Palace. He finished that campaign having featured in 25 games, earning him a call up to the Republic of Ireland Under-17's squad and was named Palace's Young Player of the Year in 2008. Following this, Scannell was offered a two-and-a-half-year professional contract with Palace. Scannell signed the contract at the beginning of the new year, and celebrated by scoring Palace's second goal in a 3–0 victory against
Wolverhampton Wanderers the following weekend. At the end of the 2008–09 season, and still only 18 years old, Scannell was voted
Championship Apprentice of the Year. He made steady progress over the following two campaigns scoring twice in each season, however he missed the first half of the 2010–11 campaign because of a knee injury, that restricted him to 19 outings in that season.
Huddersfield Town It was reported on 22 June 2012 that newly promoted
Huddersfield Town were poised to make Scannell the club's first signing of 2012 since winning promotion to the Championship via the League One play-offs. The reports stated that Scannell who was linked with the Terriers in January 2012, had agreed a three-year deal for an undisclosed fee and would move subject to a medical. Later that day, Scannell signed a three-year contract with Huddersfield, making him
Simon Grayson's first permanent signing for the club. He made his league debut in the 1–0 defeat by
Cardiff City at the
Cardiff City Stadium on 17 August. On 3 November 2012, he scored his first goal for the club in a 1–0 victory versus
Bristol City. On 18 August 2017, Scannell joined Championship club
Burton Albion on loan for the 2017–2018 season. The loan was cut short on 11 January 2018 due to an injury.
Bradford City On 19 July 2018, Scannell signed for
League One club
Bradford City on an initial two-year deal for an undisclosed fee.
Blackpool After one year at
Valley Parade, Scannell joined
Blackpool on a free transfer. He was released in June 2020.
Grimsby Town On 2 September 2020, Scannell signed a two-year deal for
League Two club
Grimsby Town. Scannell's debut season with Grimsby was hit by a string of injuries for
Ian Holloway's side, this limited him to 13 appearances in all competitions without scoring. Scannell scored his first goal for Grimsby in a 2–0 away victory over
Yeovil Town on 10 March 2022, Mariners boss
Paul Hurst commented that Scannell had been on the edges of the first team all season but due to injury and not being selected he had found it hard to include him, but equally praising his effort in training and for taking his opportunity. He went on to feature as a regular in the Grimsby team before rupturing his Achilles in a game against
Maidenhead United. Grimsby secured promotion with victory in the
play-off final, though Scannell was not in the matchday squad at
London Stadium. On 8 July 2022,
Paul Hurst announced Scannell had extended his contract by signing a new short-term deal, confirming the club would continue to support him as he continue his lengthy rehab following his injury. On 5 January 2023, Hurst announced that Scannell had extended his contract until the end of the 2022–23 season, having recovered and returned from his latest injury. On 9 May 2023, it was announced that Scannell would not be retained for the 2023–24 season and would be leaving the club when his contract expires on 30 June.
Hornchurch On 12 July 2023, Scannell signed for
Isthmian League Premier Division club
Hornchurch. The season ended in success as Hornchurch were promoted to the
National League South as champions. After nearly 100 appearances for the club in all competitions, he left the club in January 2026 to pursue an opportunity outside of football. However, several days later, Scannell was announced as a new signing for
Gibraltar Football League side
Glacis United.
Cray Valley Paper Mills In March 2026, Scannell returned to England, joining
Isthmian League Premier Division club
Cray Valley Paper Mills. ==International career==