Kearney was born and raised on a dairy farm in the Cooley Peninsula in County Louth. Like many of his peers, he got involved in athletics at an early age. The dominant sport in the area is Gaelic football. As a youth, Kearney played Gaelic for Naomh Mhuire, and in the Cooley Kickhams underage setup, before graduating to the Cooley senior football team at the age of 17 in 2004.
Kearney attended University College Dublin where he played for the rugby team. In 2005 he helped the U-20s team win the McCorry Cup, beating Dublin University Football Club in the final.
Leinster
Kearney played for Leinster at both schoolboy and U-19 level before going onto represent them as a senior. He scored a hat-trick of tries on his debut for Leinster in a pre-season friendly win over Parma. He made his Celtic League debut for Leinster in 2005 in a 22–20 defeat away to the Ospreys. He made 32 appearances in the competition, scoring eight tries, with three penalties during a period in September 2006, when usual place kicker, Felipe Contepomi was injured. Kearney played in his first Heineken Cup game in a 19–22 defeat against Bath at the RDS later that same season. It was the first of 10 appearances scoring ten tries in the process. He was part of Leinster's Heineken Cup winning team in 2009 but missed the 2011 final due to injury. On 21 May 2012, the day after Leinster won their second straight Heineken Cup he was announced as the ERC Player of the Year for 2012. During the 2011–12 campaign he started all nine European matches scoring six tries.
International career
Ireland
Kearney represented Ireland at schoolboy, U-19 and A level before he was called up to the senior squad for the 2008 Six Nations Championship. He was first called into the Irish training squad for 2005's autumn internationals and toured with Ireland Ain the summer and played in the Churchill Cup. In April 2006, he was named in the Irish squad to tour Argentina in the summer and earned his first cap on 2 June 2006. During the 2008 Six Nations Championship he scored two tries, one against Scotland and one against England. He was a member of the victorious Ireland team that won the 2009 Six Nations Championship, Triple Crown and Grand Slam. One of the most famous incidents in Kearney's career was a high tackle incident involving Italy's Andrea Masi in the first minute of a game. The incident was taken as an indication of the danger posed by Kearney in attack. In the 2011Rugby world cup Kearney was selected but was injured for the first game against the United States; however, he played in all the other matches which took Ireland through to the quarter-finals, where they were knocked out by Wales, 22–10. Kearney was selected in the Ireland squad for the 2012 Six Nations Championship and named in the starting team to play Wales in the opening match. He played in all the other games, which saw Ireland finish 3rd in the table. Kearney was also in the first Irish rugby team in 39 years to beat Australia on Australian soil, in the 2018 summer series. Kearney was named to the Ireland squad for the 2019 Rugby World Cup in Japan, where he started three of Ireland’s five matches, splitting time at fullback with Jordan Larmour.
Kearney was named in the British and Irish Lions squad for the 2009 tour to South Africa. He made his Lions test debut as a substitute in the 26–21 first test defeat in Durban. Due to an injury to Lee Byrne, Kearney was selected again for the second test in Pretoria. He scored the only try for the Lions, however they lost 28–25. He then played in the final test in Johannesburg which the Lions won 28–9. On 30 April 2013, Kearney was named to his second British and Irish Lions squad.
Kearney is the second oldest out of the four Kearney children. He has an older brother Richard, a younger brotherDave and, the youngest of the four children, a sister Sara. His brother Dave also plays for Leinster. In February 2015 he was announced as the newest brand ambassador for Newbridge Silverware.
