Watford Kiernan was born in
Rickmansworth, Hertfordshire, and began his football career as a youngster with
Watford. Watford sent two youth squads to the
Milk Cup in Northern Ireland in the summer of 2008. Kiernan was moved from his natural position of central midfielder to play centre-half, to take advantage of his height and physical presence, and also captained the side. His performances drew praise from his then coach
Mark Warburton. Later that summer, Kiernan was called up by the
Ireland under-19 team for a four-nation tournament, but he had to withdraw after tearing his right thigh muscle. In October 2008, he, together with fellow second-year scholars
Billy Gibson and
Ross Jenkins, signed "developmental contracts" – professional contracts signed earlier than usual, allowing the players to continue their scholarships without the pressure of worrying about the future –to run until the end of the
2009–10 season. When
Brendan Rodgers was appointed Watford manager in November 2008, Kiernan began to take part in some first-team training sessions. He was included in the senior matchday squad for the first time on 24 January 2009 for an
FA Cup fourth-round tie against
Crystal Palace at
Vicarage Road, and made his debut as a 90th-minute substitute, replacing
Gavin Hoyte, as Watford won 4–3. Although he made no more first-team appearances that season, he finish with 17 starts for the Academy side and 9 for the reserves.
Loans After a trial in January 2010, Kiernan joined
Scottish Premier League side
Kilmarnock on loan; manager
Jimmy Calderwood wanted him as cover with two defensive players injured. He joined Kilmarnock on the same day as Watford teammate
Scott Severin, and was given the number 29 shirt. Kiernan made his debut the next day as a 73rd-minute substitute in a 1–0 league win against
Celtic. Kiernan played three matches for
Yeovil Town at the start of the 2010–11 season,
Conference side
Histon had previously attempted to sign Kiernan on loan.
Wigan Athletic At the end of the 2010–11 season, Kiernan rejected Watford's offer of a new two-year contract; the "slightly reduced" terms offered entitled him to leave the club without compensation becoming payable by his next employer. He signed for
Premier League club
Wigan Athletic, In March 2012, having failed to feature in Wigan's first team, Kiernan joined League Two
Accrington Stanley on loan for the remainder of the season, but ankle ligament damage cut the loan spell to just three matches. He returned to match fitness with the help of a month's loan at
Burton Albion, also of League Two, in the early part of the following season, and in November moved on to League One
Brentford on loan until January 2013. He then renewed his loan deal on 11 January 2013. On 12 October 2013, Kiernan joined
League Two side
Southend United on a three-month loan deal, however, he was recalled on 29 December, as cover after
Thomas Rogne was injured on
Boxing Day. Kiernan made a strong end to the
2013–14 season, scoring his first goal for the club against
Leicester City and featuring in the play-off semi-final defeat by
Queens Park Rangers, and signed a three-year contract extension in May 2014. However, after struggling to feature the following season, Kiernan joined then Championship club
Birmingham City in February 2015 on loan until the end of the 2014–15 season. He made his debut on 21 February at
Brighton & Hove Albion, starting the match in central midfield in place of the injured
David Davis, and played 78 minutes of the 4–3 defeat. He went on to form a partnership at centre-half with
Jonathan Spector, and then played alongside
Michael Morrison after the latter's return from injury. Kiernan was credited with his first goal for the club on 11 April against
Wolverhampton Wanderers; he scored from close range after
Jonathan Grounds' shot had already crossed the line unnoticed by the officials and been pushed back into play. A hamstring injury meant he missed the last two matches of the season.
Rangers On 22 June 2015, Kiernan signed a two-year deal at Rangers with the transfer fee reported to be £200,000. Over the course of the subsequent season, he played in central defence alongside
Danny Wilson. The pair, however, received criticism for some of their performances, On 19 July 2016, Kiernan extended his contract with Rangers for another year, until 2018. Kiernan scored his first goal for Rangers and only his third goal of his professional career on 10 December 2016, heading a
James Tavernier free-kick to net the first of two goals as Rangers won 2–0. On 27 December 2016, he was linked with a £2m move to
Fulham and the following day, during Rangers' 1–1 draw with
St Johnstone, Kiernan appeared to strike Saints captain
Steven Anderson. For this, Kiernan was offered a two-match suspension for violent conduct by the
Scottish Football Association, however he chose to contest the charge and it was ultimately dismissed by disciplinary tribunal on 12 January.
Return to Southend United Kiernan returned to English football in 2017, signing a three-year contract with League One club Southend United on 3 August for an undisclosed fee. Kiernan was released by Southend on 31 January 2020.
Orange County SC On 7 February 2020, Kiernan joined USA club
Orange County SC. He made 47 appearances and scored two goals before retiring from football in April 2023, ==International career==