Munster head coach Anthony Foley died overnight at the rugby team's hotel in Paris. He was 42.
No further details of Foley's death were given by Ireland's Rugby Federation in a statement on Sunday.
"It is with deep regret that the Irish Rugby Football Union and Munster Rugby must advise of the passing overnight of Munster head coach Anthony Foley," the statement said. "Munster Rugby management are liaising directly with Anthony Foley's family and will provide them with any assistance and support required."
Munster was to play Paris-based Racing 92 in the European Champions Cup on Sunday afternoon, but the match was postponed after Munster requested that it be rescheduled.
"Munster Rugby thank Racing 92, EPCR, broadcasters and partners, and the many fans who travelled to Paris for their understanding and support," the IRFU said.
"The IRFU and Munster Rugby pass on our deepest sympathies to all of Anthony's family and friends and ask for privacy for the family at this sad time."
Foley had a successful career as a backrow forward, playing 62 times for Ireland and in two World Cups. He played more than 200 games for Munster, captaining the side to European Cup success in the final against Biarritz 10 years ago.
Still can't believe the tragic news! Thoughts to Anthony Foleys family, friends & @Munsterrugby boys— Dan Carter (@DanCarter) October 16, 2016
Thinking of Anthony Foley family & @Munsterrugby during this difficult time, our prayers are with you all.— Joe Rokocoko (@J_Rokocoko) October 16, 2016
He became head coach two years ago, having previously coached the forwards.
European Rugby Challenge Cup president Simon Halliday said Foley, who made 86 appearances in European competition for the Irish province, "was a magnificent player and a superb coach and he will be sorely missed."
Racing 92 offered its sympathies and said it fully understood Munster's request not to play.
"We share the grief of his loved ones and of the club and offer them our total support," Racing said in a statement. "We understand that in such circumstances Munster is not able to come to Colombes to play this European Cup match ... and we accept without hesitation to move it to a later date."
Foley's former Munster and Ireland teammate Ronan O'Gara " a standout flyhalf who spent his entire playing career at Munster " is now part of Racing's coaching staff.
They played together for more than 10 years at Munster.