The deal will end the Celtic League's five-year search for a sponsor.
Numerous firms have been linked with the league since its inception in 2001, but the low priority given to it by the Irish sides has dissuaded them.
But with Ireland's successes last year, those fears have been dispelled.
The three Irish sides took the top three places in last season's Celtic League, with Ulster as champions, whilst Munster's Heineken Cup triumph generated huge interest.
Magners already sponsor Celtic League side Edinburgh Gunners and Guinness Premiership outfit London Wasps.