Johnston first played club cricket in Ireland in 1995 and returned for four more seasons, meeting his former wife Vanessa, with whom he had two children. He continued playing Sydney Grade Cricket for North Sydney Cricket Club, but qualified for an Irish passport and then-coach Adrian Birrell decided he might be a useful addition to the Irish side. He made his debut for Ireland on 5 May 2004 against Surrey in a C &amp; G Trophy match that Ireland won, only their second win against a first-class county. He also played in the next match of the competition against Northamptonshire. Johnston's next appearances for Ireland were two matches against the West Indies in June, the second of which Ireland won. He then played against the MCC later that month, before making his first-class debut for Ireland against the Netherlands in the Intercontinental Cup. This was followed by the 2004 European Championship and a first-class match against Scotland. The main action for Ireland in 2005 was the 2005 ICC Trophy, hosted in both Northern Ireland and the Republic of Ireland. Johnston was in the Irish squad and they reached the final, losing to Scotland, but still qualifying for the 2007 World Cup.