Born in Kilkenny, Drennan began his career in hometown's Evergreen FC, playing for the youth side until before moving to Aston Villa in June 2010, after being tracked by Villa and Manchester City since January. In his first season, he impressed for the Academy side, scoring ten goals, and signed a professional deal in March of the following year. Drennan appeared 13 times for the reserves in 2011–12 season, also appearing four times in the NextGen Series and scoring a brace against Rosenborg. In the 2012–13 season, he played a key part for the Reserves both in U21 Development League and in NextGen Series, also being the second top goalscorer for the NextGen, despite being injured for the latter stages of the tournament. His foot injury, kept him sidelined for nine months.
Carlisle United Loan
On 17 January 2014 Drennan joined Carlisle United on a one-month youth loan deal, and made his debut as a substitute in a 2–4 home loss to Colchester United the following day. He returned to Villa in February after appearing six times for Carlisle, starting in three matches.
On 30 January 2015, Drennan returned to his home country, signing for League of Ireland side, Shamrock Rovers on a two-year deal. His first season saw him score 12 goals in 35 games across all competitions, including his first appearances in the UEFA Europa League.
Time Out Of Professional Football
In 2016 Drennan took a break from football with depression, taking work in his native Kilkenny and playing for the local hurling side James Stephens GAA and local junior football side Evergreen where he began his career, he stated he might return to Shamrock Rovers in January 2017. However, Drennan continued playing for Evergreen on their run to the FAI Junior Cup final, including a goal in the quarter-final against Kilmallock.
Sligo Rovers
Drennan returned to professional football on 13 June 2018, signing for Sligo Rovers under manager Gerard Lyttle. Drennan scored an impressive 9 goals in 17 appearances in all competitions, including a hattrick against Livingston in the Scottish Challenge Cup.
St Patrick's Athletic
On 12 December 2018, it was announced that Drennan had signed a 2-year contract with Dublin club St Patrick's Athletic, alongside former teammate Brandon Miele. He started off life at the Saints in excellent form, scoring the winner on his debut at home to Cork City, as well as scoring an injury time winner in his second game, away to his former club Sligo Rovers. On 29 March 2019, he was shown a straight red card against Bohemians for making comments to the referee after a soft free kick was given against him, he was suspended for 3 games as a result. Drennan's sixth goal of the season came on 26 April 2019, this proved to be his last goal of the season as he went on a disappointing 19 game goal drought across all competitions until the end of the season. Drennan featured in both legs of his sides UEFA Europa League tie against Swedish side IFK Norrköping. On 27 July, Drennan received another straight red card for making comments to the referee following a free kick being given against his side. This saw him receive a 4 match suspension. It was announced on 6 November 2019 that Drennan had been released from his contract a year early by new Saints manager Stephen O'Donnell, having failed to impress him during his 7 games in charge.
Return to Evergreen
Drennan left the professional game again in January 2020, opting to sign for his local Kilkenny side Evergreen once again.
Career statistics
Professional appearances – correct as of 31 October 2019.
