The 29 million euro EuroMillions jackpot was won by a syndicate from the west of Ireland.
Lottery chiefs revealed the group collected their cheque in the offices in Dublin and chose to keep their identities secret.
A spokesman for the group said the life-changing impact of the win is only just sinking in.
"This has come as a huge shock but we are thrilled. This will certainly make life easier for us all and will secure our futures," one of the winners said.
The syndicate only began to get word of the huge jackpot win when one of the members checked the EuroMillions' numbers last Saturday.
"It has been a roller coaster week with lots of butterflies in our stomachs and lots of different emotions," one of the winners said.
"But we are keeping level heads and we will try to take it all in and we will give it time before we decide what to do with our winnings."
The winning quick pick ticket was purchased from Umesh Kumar's Garryduff XL Store on Pound Road in Castlebar Co Mayo last Friday.
National Lottery chief executive Dermot Griffin said he was delighted to hand over the winning cheque to the syndicate.
"It is important these winners take the time and space to let the news sink in," he said.
"This is an incredible amount of money to win, and we have recommended that the winners get independent financial and legal advice."
It is the second EuroMillions jackpot win for Castlebar.
In April 2014, a jackpot prize worth a massive 15 million euro was claimed on a ticket sold in Staunton's Costcutter Express in Castlebar. On that occasion, the winner did not go public.
It is also the 11th time a EuroMillions jackpot has been won in Ireland.
It follows the win for a Dublin work syndicate in January this year when they shared out an 88.5 million euro jackpot.
The country's biggest win remains Dolores McNamara from Limerick, who took home 115 million euro in 2005.