Tierney spent his youth career with St. Kevin's Boys (which was amalgamated with Bohemians in 2018) from the age of eight.[1] Having progressed through the Bohs-SKB U17s and U19s teams, Tierney came to prominence in the second half of the 2019 season with his displays.[citation needed]
He made his first-team debut in the League of Ireland Cup Quarter Final clash with Cork City in May 2019, scoring the opening goal in a 2–0 win.[2] He struck a further three times during his rookie season, hitting the net against UCD,[3]Dundalk[4] and Waterford.
On 1 December 2021, Tierney signed with Motherwell for an undisclosed fee on a three-and-a-half-year deal.[7] He completed the move on 1 January 2022. Tierney made his debut for the club on 22 January 2022 in a Scottish Cup tie against Greenock Morton, coming off the bench in the 76th minute and winning a penalty as his side won 2–1 in extra time. His first goal for Motherwell came on 1 February where he netted a 92nd minute equaliser in a 1-1 draw against St Mirren.
His second goal came at home to Rangers.[8][9]