[1] At the Ballyfermot club Markey played alongside future League of Ireland players Max Kouogun, Josh Collins and Derek Daly among others.
Markey's form for the Orchard attracted interest from St Patrick's Athletic, who signed him for their under 19's side ahead of the 2014–15 season.
[5] He made his first team debut on 9 February 2015, aged just 17 years old, when he came off the bench in a 7–0 win at home to Tolka Rovers in the Leinster Senior Cup.
[6] Markey made late substitute appearances in the President's Cup away to Dundalk and the first game of the 2015 League of Ireland Premier Division season away to Shamrock Rovers, his first Dublin Derby.
[15] His performances in these games stood out and earned him plaudits from fans, pundits and most importantly manager Liam Buckley, who kept him in the starting 11.
[16][17] He went on to make a total of 18 league appearances, 22 in all competitions and his form in those games saw him voted as St Patrick's Athletic Young Player of the Year by the club's fans.
[22] Markey had to wait until 15 March 2019 for his first appearance of the season under new manager Harry Kenny, coming on as a substitute in a 2–0 loss away to Waterford.
[25] Markey saw his first taste of European football when he came off the bench away to IFK Norrköping of Sweden in a UEFA Europa League Qualifier, assisting Conor Clifford's goal in a 2–1 loss for Pat's.
[32][33] On 14 March 2022, he sustained a broken foot in a 2–0 defeat to Derry City at the Ryan McBride Brandywell Stadium on just his 5th appearance of the season, with the injury requiring surgery.
[35] He had to wait until the final day of the season for his first league goal of the year, opening the scoring in a 2–0 win over relegated Finn Harps on 6 November 2022.