A native of Clondalkin, County Dublin, Shields started playing youth football for St Francis at the age of seven until he joined League of Ireland side Bray Wanderers at the age of 17, initially playing for their reserve team. He made his senior debut on the last day of the 2008 season aged 17 and then established himself as a regular in the first team in the 2009 season. Shields was part of a Bray side that battled to preserve its Premier Division status during his time with the club. The highlight of his time with Bray was an extraordinary promotion/relegation play-off match with Monaghan United in November 2010. Shields scored an own goal in the 119th minute which saw his side fall behind. Celebrations from Monaghan fans resulted in a perimeter wall collapsing and a long delay. When play resumed, Shields set up an equalizing goal for Bray and he then scored the winning penalty kick in the shoot-out that followed.

After three goals in 90 appearances for Bray Wanderers, Shields moved to Dundalk ahead of the 2012 season. The then-Dundalk manager Sean McCaffrey made Shields the captain of the club and he’d go on to become a legend, leading the club through the most successful period in their history. However, in his first season with Dundalk, the Lillywhites only avoided relegation due to Monaghan going out of business. Shields was then one of just two players to be kept on the following season as Stephen Kenny came in as manager and ignited a golden era. The central midfielder would go on to win five League of Ireland titles, three FAI Cups, three League of Ireland Cups, three President’s Cups, one Leinster Senior Cup, and one Champions Cup – against Linfield, incidentally. During his nine years at Oriel Park, Shields scored nine goals in 249 appearances. Along with the success, he also qualified for the group stages of the UEFA Europa League on two occasions (2016 & 2020) with Dundalk.

With Shields having just six months remaining on his Dundalk contract in 2021, Linfield were permitted to contact the experienced midfielder and subsequently an agreement was reached between the clubs for a small fee to be paid to Dundalk which allowed Shields to join Linfield on a three-year contract in the summer of 2021 and be part of the Blues’ European campaign, rather than holding off until January 2022.

Shields had lived in Bangor for a lot of his time as a Dundalk player, and through this he got to know Blues boss David Healy and captain Jamie Mulgrew, and he said this was a factor in him choosing Linfield amidst interest from rival Irish League clubs.

He arrived at Windsor Park at the age of 30 and instantly hit the ground running, sweeping up the individual awards in his first season as he was named Ulster Footballer of the Year, NIFWA Player of the Year and Linfield Player of the Year, as well as being named in the NIFL Premiership Team of the Year. His strong performances in the middle of the park in his debut season were key in the Blues’ title success, particularly on the defensive side of things as he protected a back four who only conceded 24 times in 38 league matches.

Shields’ first Linfield goal came in his second match and his home debut – a penalty in a 2-1 defeat to Romanian side Zalgiris Vilnius at Windsor Park in a Champions League qualifier. He quickly established himself as Linfield’s penalty taker and would go on to score seven goals in his debut season.

In his second season at Windsor Park, Shields went one better as he scored eight goals in 53 appearances. He was seconds away from booking a place in the group stages for what would have been the third time in his career, only for Linfield to suffer late heartbreak against RFS in the play-off round of the UEFA Conference League. He won the League Cup in his second season, scoring the second goal with a penalty in the Blues’ 2-0 win over Coleraine in the final.

Shields made another 36 appearances in the 2023-24 season, although had spells on the sidelines due to injury. He picked up a knee injury in a League Cup Quarter-Final away to Larne on December 5, 2023, and it kept him out for two months, returning in February and playing a further four matches before a reoccurrence of the injury. This time it was expected that the injury would end his season, but against the odds he returned just in time for the split fixtures. Although most of his appearances were in central midfield, he played a number of matches as a defender in a back three. He added another League Cup medal to his collection with a 3-1 win over Portadown, but was on the losing side to Cliftonville in the Irish Cup Final.

He signed a two-year extension in January 2024 which contracts him with Linfield until the end of the 2025-26 season.