McDermott began his career at Ashbourne United then moved to Cherry Orchard of Dublin, where he was scouted, and signed up by Manchester City. He was a member of the team that clinched the 2008 FA Youth Cup for the Blues. In September 2008, McDermott joined Football League One side Milton Keynes Dons on a one-month loan deal, making his début against Oldham Athletic. This would turn out to be his only appearance for the MK Dons as his loan spell was ended prematurely due to injury. McDermott returned to Manchester City where he remained in the reserves for the remainder of the season. At the start of the following season he signed a new two-year deal to stay at the City of Manchester Stadium. After signing the new contract, McDermott joined Football League Two side Chesterfield on a three-month loan deal. He made his debut on the opening day of the season in a defeat to Torquay United, before scoring his first goal for the club on his home debut in a 1–0 win over Northampton Town. Towards the end of the January 2010 transfer window McDermott went out on loan for a third time, this time until the end of the season at Championship side, Scunthorpe United. For the first seven months of the 2010–11 season Donal continued to play for the Manchester City Elite Development Squad before going out on loan for a fourth time on 24 March 2011, this time to Football League One club Bournemouth. He helped the Cherries reach the Play-offs following a sixth-placed finish by scoring against Huddersfield in the first leg but they were defeated on penalties by Huddersfield on the second leg. That was his only goal on loan at AFC Bournemouth.
On 1 July 2011, he signed a two-year contract with Huddersfield Town on the expiry of his contract at Eastlands. As he was offered a new deal, the fee will be decided by a tribunal. He made his Terriers debut in the 1–1 draw against Bury at the Galpharm Stadium on 6 August 2011. He scored his first goal for the club in the 2–1 win over Northampton Town at the Sixfields Stadium in the Football League Trophy on 30 August 2011.
AFC Bournemouth
On 31 January 2012, McDermott made a permanent switch from Huddersfield to return to Bournemouth.
On 7 May 2015, McDermott signed a one-year contract with Rochdale.
Swindon Town
On 13 July 2017, McDermott joined League Two side Swindon Town on a one-year deal following a successful trial. On 12 August 2017, McDermott made his debut for Swindon during their 1–1 home draw with Exeter City, featuring for 45 minutes before being replaced at half-time by Kaiyne Woolery. A week later, McDermott scored his first Swindon goal in their 1–0 away victory against Morecambe, firing his effort past goalkeeper, Barry Roche in the 13th minute. He was released by Swindon at the end of the 2017–18 season.
