It is following a swimming accident in Spain yesterday.
In a statement, Tyrone GAA says "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."
Tyrone GAA chairman Michael Kerr added: "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."
In total, Casey scored over 1,000 points across league and championship matches.