Zirnis made his debut for Latvia national football team on 19 August 1997 in a 0–0 draw with Azerbaijan. In November 2003 he was a part of the Latvian team that beat Turkey over two legs to qualify for the UEFA Euro 2004 finals, for the first time qualifying for a major international tournament. and played at the finals in 2004. Zirnis was sent off in the Baltic Cup final against Lithuania in Kaunas on 22 May 2005. On 17 November 2007 he scored an own goal to give Liechtenstein the lead in a UEFA Euro 2008 qualifying Group F match which Latvia eventually won 4–1. Zirnis played 68 international matches for Latvia.