He made his professional debut in April 2011 after impressive performances with the reserve team led manager Nigel Clough to include him in the senior setup.

He quickly became a regular starter and appeared in the 2014 Championship play-off final where Derby missed out on promotion with a 1–0 loss to Queens Park Rangers.

His time at Newcastle was interspersed by loan spells at Championship clubs Queens Park Rangers, Reading and Sheffield Wednesday respectively.

Hendrick joined Derby County as a youth player in 2008 from St Kevin's Boys in Dublin, having overcome Osgood-Schlatter disease,[4] and played regularly for the club's academy side before breaking into the reserve team during the 2009–10 campaign.

[13] Despite missing the majority of Derby's pre-season due to his call-up to the European under-19 tournament,[12] Hendrick was involved in first-team duty from the start of the campaign, making substitute appearances in the season's opening two matches, against Birmingham City[14] and Watford.

[22] Hendrick missed Derby's Christmas and New Year programme of fixtures with an ankle ligament injury in a 1–0 defeat against Ipswich Town.

Hendrick began the 2012–13 season in the starting eleven but after poor performances and scoring only once in his first 16 matches, he was dropped for Michael Jacobs in the 3–2 victory over Birmingham City on 24 November 2012.

[37] Ahead of the 2013–14 season, Hendrick was set to compete with Craig Bryson, Paul Coutts and Will Hughes for a starting berth, with Clough stating that rotation could be utilised.

[38] He started in Derby's first five matches, before succumbing to a chipped ankle bone injury in the 3–0 win at Yeovil Town in late August, with a scan revealing damage worse than expected[39] and was later ruled out for 12 weeks.

[47] Derby needed a victory in their final match against Reading to secure a play-off place but lost 3–0 and finished the 2014–15 season in eighth position as ultimately they had earned just 12 points out of a possible 39 since beating Charlton Athletic in February.

[56] On 31 August 2016, Hendrick joined Premier League side Burnley for a club record £10.5 million fee on a three-year deal.

[58] He made his first start for Burnley on 17 September, playing the full 90 minutes in a 3–0 away defeat against reigning Premier League champions Leicester City.

[66] He netted his second goal of the season in a 1–0 home victory over Newcastle United on 30 October in what was Sean Dyche's fifth anniversary as Burnley manager.

[78] On 14 September 2019, Hendrick scored in his 100th Premier League appearance for Burnley with a stoppage-time equaliser in a 1–1 draw at Brighton & Hove Albion.

[79] It had appeared that Hendrick was in Sean Dyche's future plans as he had started 22 of Burnley's 29 matches before the 2019–20 season was brought to a sudden halt because of the COVID-19 pandemic.

[83] He described Newcastle as 'a massive club' and manager Steve Bruce hailed his new signing by saying: "He’s an experienced international footballer who can play all across the midfield so we welcome him aboard.

[84] Two weeks later, Hendrick scored and also set up a goal for Callum Wilson on his competitive debut for Newcastle in a 2–0 away win over West Ham United.

[86] On 6 February 2021, Hendrick was sent off early in the second half of a 3–2 home win over Southampton for a second bookable offence after pulling back Takumi Minamino.

[91] After finding his game time severely limited at Newcastle, on 31 January 2022 — his 30th birthday — Hendrick joined Championship side Queens Park Rangers on loan until the end of the season.

[92] Five days later, he made his QPR debut in a 2–0 FA Cup defeat away at Peterborough United, coming on as a substitute at the start of the second half.

[96] On 12 July 2022, Reading announced the season-long loan signing of Hendrick where he would link up with compatriot and fellow newcomer Shane Long.

[104] On 14 January 2023, Hendrick netted two first-half goals in a 2–2 draw at home to former club Queens Park Rangers — his first brace since February 2015.

[106] This was coupled with the club being handed a six-point deduction towards the end of the season over a historical breach of the EFL's profitability and sustainability limits.

[108] After being sent to train with Newcastle's under-21s, Hendrick signed for Championship club Sheffield Wednesday on 1 September 2023 on a season-long loan deal.

[110] His first goal came on 29 November, scoring a stoppage time equaliser in the 93rd minute against top of the league Leicester City in a match that ended 1–1.

[122] Hendrick came on as a 71st-minute substitute for James McCarthy in the 2–0 win and provided an assist for Wes Hoolahan for Ireland's second goal in this match, showing great composure in doing so with a well-executed pass.

[123] Hendrick received his first Ireland start in his fourth cap, in a match against World and European Champions Spain on 12 June 2013 at the Yankee Stadium in New York City.

He opened the scoring in the third minute in the 2–2 away draw against Serbia in a 2018 World Cup qualifying match with a low shot just outside the penalty area which was deflected in off Branislav Ivanović.

[139] Owing to his lack of proficiency in scoring goals, Hendrick's most frequent task is recycling possession high up the pitch to create chances for his teammates.

[139] Growing up, Hendrick played Gaelic football for Parnells and has a close friendship with Dublin county team goalkeeper Stephen Cluxton.