Murphy went on to make over 100 appearances for Sunderland, whilst also spending time out on loan with Sheffield Wednesday and Ipswich Town, before joining Scottish side Celtic in 2010.

[7] He played youth football for local side Southend United before joining English club Luton Town on 14 November 2000.

[11] In the 2004 season, he notched 14 league goals in the Premier Division which earned him the PFAI Young Player of the Year award.

[13] He made his premier league debut in a 1–1 home draw with West Ham United at the Stadium of Light on 1 October 2005.

[14] Murphy had a one-month loan spell at Sheffield Wednesday during the 2005–06 season, making 4 appearances before being recalled by Sunderland.

[15] On 12 February 2006, he scored his first Premier League goal after coming off the bench at home to Tottenham Hotspur, equalising with minutes to go.

[15] He netted one goal each on the opening day of the season against Coventry City and on 12 August against Plymouth Argyle though both matches were lost.

Murphy scored his third goal of the season in the Black Cats' 2–1 away win over Queens Park Rangers on 28 November,[17] and then notched in successive games against Norwich City and Luton Town.

[13] On 26 April 2008, Murphy came on as a substitute and scored the winning goal in a 3–2 victory over local rivals Middlesbrough, securing Sunderland's status in the Premier League for the following season.

[24] On 1 February 2010, Murphy signed for Ipswich Town on loan for the remainder of the 2009–10 season, once again joining up with Roy Keane, his former manager at Sunderland.

[26] He then scored the crucial second goal at Queens Park Rangers on 9 February,[27] and another in his third appearance against Peterborough, making his record 3 in 3 for Ipswich.

[33] Murphy made his competitive debut for the club on 28 July 2010, coming on as a second half substitute in a 3–0 away defeat to Braga in a third-round UEFA Champions League qualifying match.

[35] The following week, Murphy scored his first goal for Celtic, netting the match-winner from the penalty spot in a 1–0 victory over Motherwell on 29 August.

[42] On 7 June 2013, Murphy agreed a two-year deal with Ipswich, finally making his move permanent after several loan spells with the club.

[51] After a comparatively slow start to the 2015–16 season in which he failed to score in his first 13 appearances of the league campaign, Murphy ended his drought by netting his first hat-trick in English football in a 2–5 away victory over Rotherham United on 7 November 2015.

[54] On 28 August 2016, Murphy signed for Championship club Newcastle United on a two-year contract for an undisclosed fee.

[62] In an interview with the Shields Gazette, Murphy said that although it was expected he would leave the club, he knew he was brought in to do a specific job, and praised Rafael Benítez for his man-management skills.

[72] On 2 September 2019, Nottingham Forest and Murphy mutually agreed to terminate his contract, allowing him to become a free agent.

[84] He scored his first goal for the Republic of Ireland in his 23rd appearance, netting in a 2–2 draw away to Serbia in a 2018 World Cup Qualifier on 5 September 2016.

[85][86] On 6 October 2017, Murphy scored a brace in a 2–0 victory over Moldova in a 2018 FIFA World Cup qualification match at the Aviva Stadium in Dublin.