The company behind the Prize Bond weekly draw is introducing a once-a-year jackpot of €1m.
The draw for the new top prize will take place each December in the run-up to Christmas.
The Prize Bond Company is also increasing the current monthly jackpot prize of €150,000 to €500,000 for the remaining 11 months of the year.
The first €500,000 draw takes place on Friday, to mark the 50th anniversary of the first draw in September 1957.
There are close to 100 million prize bonds held, with a total value of around €590m.
There is over €1.2m in unclaimed Prize Bond winnings.
The biggest single outstanding prize is €20,000, but most of the others are much smaller amounts.
The money never goes back into the fund, but rests with the Prize Bond Company indefinitely until it is claimed.