50-year-old Paddy Millar used birthdays and anniversary dates as his numbers in last Friday's (August 10) draw, reports PA.
He incorrectly used the number 48 for the age of his wife Gail when in fact she is 49.
Millar claims Gail was certainly not complaining when she realised the mistake and has forgiven him, especially considering he made her younger.
He added: "It's the best mistake I've made in my life."
The couple will celebrate with a holiday with their children, Alex, 17, and Julia, 11, but only across the Irish border for a four-day break. However, they plan to "go further afield in the not-too-distant future".
The pair have secured the £1,994,322.90 prize alongside Adrian and Gillain Bayford, who are celebrating the second-biggest UK lottery win after taking more than £148 million.
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