Doyle signed for his first professional club St Patrick’s Athletic in September 2001 as a free agent. Doyle initially played for the club’s under 18 side but within weeks had made his League of Ireland début as a substitute on 24 September at Oriel Park.

He then had a successful spell with Cork City, who he joined in February 2003, linking up with his former St. Patrick’s Athletic manager Pat Dolan, who had recently taken over at Turners Cross. Doyle scored 25 goals for Cork City where he began playing on the right-wing but soon reverted to his natural position of striker. He also scored two goals from six appearances in the 2004 UEFA Intertoto Cup, including the winner against Dutch side NEC Nijmegen. Thousands of fans gave him a standing ovation in his final game for Cork City, against Finn Harps, before his transfer to Reading.

Doyle played for the Republic of Ireland U21 team, making his debut on 2 February 2004 against Portugal. With 11 caps, and scoring six goals making him joint top scorer at that grade, he was also a member of the Irish FIFA World Youth Championship squad in 2003. Scored against Colombia. He was called up to the senior Republic of Ireland squad in October 2005, making his full international debut against Sweden at Lansdowne Road on 1 March 2006. His first start in a competitive international was against Germany on 2 September 2006 in a Euro 2008 qualifier. With this appearance, his family also won a €100 bet at 100/1 they had placed two years earlier on Doyle playing competitively for Ireland.

On 7 June 2005, Doyle signed for Reading on a two-year deal from Cork City, Ireland for €117,000 (about £78,000). Despite having left Cork City, Doyle received a championship medal when they won the League of Ireland Premier Division in November 2005. Originally, he was signed as a back-up, but injuries to Dave Kitson and then Leroy Lita gave him an opportunity in the first team which he retained thanks to a number of important goals along the way. He scored 19 in total including the equalising goal at Leicester City on the day the Royals were promoted to the Premier League for the first time in their history, 25 March 2006. He became an intrinsic part of Reading’s Championship-winning 2005–06 squad, and according to the Actim Index was the top player in the Championship. He was named the official Reading F.C. player of the season for 2005–06. In April 2006, he was also named as Championship Fans’ Player of the Year and one of the Championship Team of the Year by the Professional Footballers’ Association (PFA).

Doyle scored his first goal for Ireland against San Marino in November 2006. He was then named the official Football Association of Ireland Young Player of 2006 on 4 February 2007, also receiving a nomination for the senior award.

His second goal came in a 1–0 victory over Slovakia at Croke Park in March 2007. He also scored against Ecuador on 23 May 2007 securing a valiant 1–1 draw against the South Americans, and he scored his best goal for Ireland, with a long-range left-foot shot into the top corner to the keeper’s right, in a 2–2 draw against Slovakia on 9 September 2007 in the 2008 European Championships qualifying game in Bratislava. His fifth international goal and fourth in the Euro 2008 qualifying campaign came away to Wales as Ireland drew 2–2.

On 1 September 2008, he was linked with a move to Aston Villa but in the end the striker remained at Reading.

Doyle joined newly promoted Premier League side Wolverhampton Wanderers on 30 June 2009 when he signed a four-year contract for a club record fee, reportedly £6.5 million. However, he immediately had to undergo an operation to remove a hernia, which hindered his first pre-season at his new club. He picked up another minor injury while on international duty with Ireland that caused him to miss the start of the league season. He finally made his debut for Wolves on 22 August 2009, as a half time substitute at Manchester City.

He scored his first Wolves goal on 20 September 2009 to earn a 2–1 win over Fulham. Throughout much of the season he operated as a lone striker in a 4–5–1 formation. His contribution helped the club finish 15th in their first season in the Premier League. He also won the club’s Player’s Player of the Season Award and their Top Goalscorer Award after reaching nine goals.

Doyle was nominated as the Football Association of Ireland Player of the Year for 2007 and won the award in 2009, as well as for Goal of the Year for his goal against Slovakia in September 2007. Doyle scored his ninth international goal, and first in 2012 European Championships qualification, against Andorra at the Aviva Stadium on 7 September 2010. On 6 February 2011, Doyle was named Republic of Ireland Player of the Year for 2010, while he also earned the Goal of the Year award for Ireland, for his superb strike in last September’s Euro 2012 qualifier against Andorra.

During 2010 World Cup qualifying, Doyle established himself as Robbie Keane’s ideal strike partner in the national team’s attacking quartet that included Damien Duff and Aiden McGeady. He played in 8 of the 10 games, scoring two goals, including the first in a 2–1 win in the opening game against Georgia. He also scored against Cyprus, his seventh international goal, in a 2–1 win in Nicosia. In the absence of Robbie Keane, Shay Given and other players who had been allowed to return to their clubs, Doyle captained Ireland for the first time in a friendly against South Africa on 8 September 2009.

Doyle continued to play the lone striker role throughout most of the 2010–11 season, despite the arrival of Steven Fletcher who equalled Doyle’s club record fee. After Mick McCarthy rotated systems using various strikers, he eventually reverted to his previously successful 4–5–1 formation, with Doyle playing the lone front man role. He went on to score five goals for Wolves in the second half of the season – including the winner against Manchester United that halted their 29 match unbeaten run. On 26 March 2011, Doyle tore a knee ligament while on international duty with Ireland, which kept him out of action for up to 6 weeks. He returned to action on 14 May against Sunderland, coming on as a substitute, but played no part in their final game in which they narrowly escaped relegation.

Doyle was sent off against Armenia on 11 October 2011. He reached 50 international caps against Italy at UEFA Euro 2012.

On 31 January 2014 Doyle was loaned to Championship side Queens Park Rangers in a deal due to run until the end of the season. He scored on his debut against Burnley on 1 February 2014, and netted a second goal for the club against Reading, as the team chased an immediate return to the Premier League. However, a knee injury sustained against Birmingham City saw him spend a period back at Wolves during March and April as he underwent treatment. After returning to Loftus Road, Doyle featured in all three games of QPR’s successful play-off campaign as they won promotion back to the Premier League.

On 1 September 2014, Doyle was loaned to Premier League side Crystal Palace until January 2015. He made only three substitute appearances for the Eagles in the Premier League, failing to score in any of these appearances. His final appearance during his loan spell saw him score his only goal for the club when he netted in an FA Cup tie against non-league Dover Athletic. At the end of his loan, he returned to Wolves, making a handful of appearances as substitute before his contract expired at the conclusion of the season. In total he scored 33 times for Wolves during 179 appearances, spread across playing from the Premier League to League One level.

On 20 March 2015 it was announced that Doyle had signed a pre-contract agreement with the MLS club Colorado Rapids. The two-and-a-half-year deal was originally due to take effect from 1 July 2015 upon the expiry of his contract with Wolves, but the club reached a settlement on 6 May 2015, following the conclusion of Wolves’ season, to allow Doyle to join Colorado earlier than planned. He made his MLS debut on 23 May 2015 as a substitute in a 1–0 win over the Vancouver Whitecaps.

On 28 September 2017, Doyle announced his immediate retirement from the game due to ongoing concussion issues.