The lucky winner of a near 40 million euro (£35 million) EuroMillions jackpot bought the ticket in the Dublin area.
Lottery chiefs said the numbers for last Friday's draw were a quick pick selection and that there has been no contact from the ticket-holder almost four days on.
"Nobody has come forward yet and we are now appealing to players in the Dublin region who bought a EuroMillions ticket last week to check their numbers to see if they are eligible to claim a cool 38,906,715 euro," a National Lottery spokeswoman said.
The winning numbers are 04, 08, 22, 23, 48 and Lucky Stars 01 and 12.
Details of exactly where and when the ticket was bought are expected to be released in coming days.
"We are urging all our players to check their tickets to see if they are the lucky winner or winners," the spokeswoman said.
"At this stage we don't know if it's a single winner or a syndicate.
"We urge anybody who has this ticket to keep it safe until they can make contact with us and get into the National Lottery office to make their claim. This is a valuable piece of paper."