The unidentified player was one of two winners who shared the 132m euros (£100m) jackpot after the draw on Friday, 29 January.
The other winning ticket was sold in France.
The 66m euros prize is the fourth largest Euromillions win in the history of the game in the Republic of Ireland.
A spokesperson for the Irish National Lottery has not revealed where the ticket was sold, but the winner has 90 days to collect the cheque in person from its headquarters in Dublin.
A Euromillions ticket holder in the Republic of Ireland who has won 66m euros (£50m) in the latest draw has contacted officials to claim the money.