It is understood Mr Casey, who was aged in his late 20s, had been in Spain attending a wedding, where he was to be a groomsman.
He passed away following an accident at a swimming pool.
Considered Tyrone’s greatest ever hurler, he played for club side Eoghan Ruadh, Dungannon.
His death was confirmed in a statement from Tyrone GAA.
It said: “Heartbreak and grief can come in many forms but are especially cruel when brought about by the sudden death of someone young, talented, outgoing and in the prime of life.
“Accordingly, Gaelic Tyrone is totally shaken by the news of the tragic death of Damian Casey, a supreme hurler just now at the peak of his powers.”
Tyrone County Chair Michael Kerr said: "This breaking news is beyond belief.
“Someone who brought honour, delight and simple, straightforward gladness to our whole county, and far beyond, is gone and it’s so hard to take in.
“There are very difficult days ahead for Sean, Susan, Louise and Catherine, for Damian’s wider family circle and many friends and associates, for his team-mates and for the Gaels of Dungannon and Tyrone.
“Our deepest sympathy goes to them, and our focus must now be on providing the support and comfort that will be so badly needed.”
Tyrone GAA added: “Damian Casey was a special gift to Tyrone and to hurling. We were the richer for his presence and are the poorer for his passing.”
Less than three weeks ago, Mr Casey had enjoyed Nicky Rackard Cup glory with Tyrone, beating Roscommon in the final at Croke Park.
It was the second time he had won the trophy, having captained the first Tyrone side to lift it in 2014, when he delivered what was described as an "inspirational” performance, scoring eight points at Croker in a shock final win over favourites Fingal.
Broadcaster Adrian Logan said "He was a top class hurler and a great lad. Everybody is clearly and utterly heartbroken, devastated and distraught.
"Going from celebrating victory in Croke Park to this. He was such a decent and good lad as well as a great athlete.”
Last month, the Belfast Telegraph reported that Mr Casey had been a vital part of the Tyrone team and over a decade had started for the team 100 times in a row.
Even more remarkable was that he had scored in every single one of the games.
His playing figures saw him score over 1,000 points in the course of his Tyrone career in league and Championship.
He was also described as “probably the most prolific scorer in hurling history”.