The ticket was bought in Derry on July 31 and the Millionaire Maker code was one of two winners in the UK.
The ticket holder, Rory McParland, travelled around Ireland for three weeks not realising it was a winner.
He heard about the unclaimed prize when he returned to Derry.
“When I got home from holiday, the rest of the syndicate were naturally eager for me to check our tickets and see if we were the lucky winners,” said Mr McParland.
“We’ve never won much before so hopes weren’t high when I took my tickets back to the store where we’d bought them if the first place.”
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The syndicate of 23 is made up of existing and former staff at the Greater Shantallow Area Partnership, a community support agency operating across the northern part of Derry city.
Each person will now pocket more than £43,000 each.
“As she checked the tickets, the shop assistant told me that we’d won £3 on one line, £5.20 on another and then £7.60 on another, which is our usual form,” added Rory McParland.
“Then she stopped and said that we were going to have to call the National Lottery about one particular line as she wouldn’t be able to pay that amount over the counter.
“I asked what she meant and all she could tell me was that we had won big money.
“Before we knew it, we were all jumping around the place, going mad, shouting and cheering. I think there were even a few tears shed.
“It was an incredible moment that I will never forget. We are all over the moon.”
Mr McParland said he loves to holiday in his touring caravan, and that he has been looking for a car for a long time.
Rory told BBC News NI: “To think that this one win will change the lives of 23 families is amazing, whether it pays for a dream holiday, helps with the mortgage or whatever each member chooses to spend it on.”