Casey is among 8 players to sign contracts with the Sky Bet Championship side, which finished three spots above the safety zone this season.
Casey, who represented Avenue Utd at all levels, had been released by Leicester City following the completion of his scholarship at the Premiership club where he featured prominently in their U17 and Youth Games over a two year period.
He follows in the footsteps of former CSSL player Alan Brooks (Lifford), who has previously played for Millwall FC.
Milwall have also signed terms with 7 other players including Sid Nelson, Ben Thompson, Keaton Wood, Denzel Gerrar, Jake Goodman, Jack Powell and Josh Siafa.
Manager Ian Holloway said: “I’ve got to find a younger balance in the first team and I think we have already got some young players at the club who can come into the group.”
The Clare District Soccer League has congratulated Casey on his success, stating: “The CSSL would like to congratulate former CSSL and Avenue Utd player Dylan Casey who has signed a professional contract with Millwall FC. Dylan and was a prominent member of CSSL ETP from U11 -U16. Dylan progressed to the FAI Regional Center Emerging Talent Programme, and received International recognition by captaining the Republic of Ireland U15 B Squad in the Tri Nations.”