The deal is expected to be completed by the middle of January.
It's the second big acquisition for Magners this year, following the deal to buy Tennent's from Anheuser-Busch Inbev.
The deal will give C&C an expanded cider portfolio that includes Blackthorn, Olde English and Gaymers, a cider production facility at Shepton Mallet in Somerset and a distribution base in Bristol.
Gaymer's current annual cider volumes of 1.5 million hl are roughly twice those of Magners, and some 80% of the Gaymer business is in the off-trade.
C&C chief executive John Dunsmore said: "The transaction strengthens our position within the world's largest cider market and broadens the scope of the group's existing cider offering."
Troy Christensen, president of Constellation Wines Australia and Europe, said: "For the UK and in Europe the drive is to focus primarily on wine.
"For Gaymer, the opportunity now exists to develop as part of a business that is focused on the cider and beer sector."
Gaymer managing director Peter Spencer added: "It is business as usual for both organisations for now, at what is a very busy time of the year."