In July 2009, Stirling scored his maiden first-class century. In Ireland's opening match of the 2009–10 ICC Intercontinental Cup, he opened the batting with Jeremy Bray and in the first innings scored 100. Stirling was a member of the Ireland squad that participated in 2010 Under-19 World Cup hosted by New Zealand in January. The team were runners-up to Bangladesh in the Plate Championship, finishing tenth overall out of 16 teams. Stirling was the team's leading run-scorer, with 209 from five matches.

In December 2009 Stirling signed a three-year contract with Middlesex County Cricket Club, joining former Ireland international Eoin Morgan at the club. According to Angus Fraser, the club's director of cricket, representing Ireland is Stirling's priority; he also stated that "Paul has been involved with the Middlesex youth sides and Second XI and we are delighted that he has now signed a full time contract for the club. His batting for Ireland in 2009 highlighted his potential". The following month Cricket Ireland, the governing body for cricket in Ireland, awarded Stirling a full-time contract. He was one of six players to be awarded such contracts with Cricket Ireland, and came just a year after the first professional contracts were awarded to Ireland's cricketers. Before that players were amateurs relying on income from other jobs and playing cricket in their spare time. The contract allowed Stirling and the others to focus on cricket, with the aim of improving ahead of the 2011 World Cup.

After the 2011 Cricket World Cup, Stirling broke into Middlesex's one-day side during the 2011 season, making his List A debut for the club on 24 April against Worcestershire. Although Middlesex failed to progress beyond the group stages of the 2011 Clydesdale Bank 40, Stirling enjoyed personal success. Scoring 535 runs from 12 matches, he was Middlesex's leading run-scorer and 7th in the competition. During an innings of 68 against Yorkshire, one of Stirling's two half-centuries in the competition, he passed 2,000 runs in List A cricket. Towards the end of the competition Stirling struck his first century in Middlesex colours, scoring 109 runs from 81 balls to a 34-run win over Derbyshire.

Although a full-strength team played against England in an ODI in August 2012, county commitments meant many of Ireland's senior players were unavailable to play in the team's opening match of the 2011–13 Intercontinental Cup. An attacking half-century from Stirling in the final innings of the match helped secure victory for Ireland. Later that same month Stirling scored his second first-class century. His innings of 107, beating his previous highest score of 100, came off just 79 deliveries and helped Ireland to their second win of the Intercontinental Cup.