He made his Meistriliiga debut on 16 July 1995, five days after his 16th birthday, against Eesti Põlevkivi in the opening match of the season.
Fredrikstad returned to the Norwegian top division after a season, with Piiroja scoring in both legs of an 8–1 aggregate win against Hønefoss in the promotion play-offs.
On 31 August 2011, Piiroja signed a one-year contract, with an option of extension for another season, at Eredivisie side Vitesse.
[8] Piiroja made his international debut for the Estonia national team on 21 November 1998, in a 2–1 loss against Armenia in a friendly, replacing Viktor Alonen in the 85th minute of the match.
He was sent off by referee Viktor Kassai in the 76th minute of the first leg of the UEFA Euro 2012 qualifying play-off against Republic of Ireland, which Estonia went on to lose 5–1 on aggregate.
Piiroja ended his international career in October 2013, but made one final appearance for Estonia on 31 March 2015, in his testimonial match against Iceland.