The competition is to be renamed the Magners League as part of the deal, which will help secure the future of the five-year-old competition.
Magners already has a relationship with rugby union via its shirt sponsorship deals with the Edinburgh Gunners and London Wasps. It has retained the sponsorship agency BrandRapport to handle strategic development and activation of the sponsorship.
The Magners League combines teams from Ireland, Scotland and Wales. It was formed in 2001 as the Celtic League, but struggled to stay afloat in its formative years.
Reorganisation has improved the league's fortunes and last year's attendance record showed a 20% increase on average. It hopes to become one of the leading competitions on the world rugby calendar.
Maurice Breen, marketing director of WM Magners, said: "The Magners League taps into the passion of fans across the UK and Ireland and our aim is to extend the reach for the competition and to enhance the fan's experience. We have an exciting vision for the future of the Magners League and we see this complementing our ambition in the UK and Ireland."
Ulster are the reigning champions. Previous winners include the Neath Swansea Ospreys in 2004/2005; the Llanelli Scarlets in 2003/2004; Munster in 2002/2003; and Leinster in 2001/2002.
Magners cider is called Bulmers in its home market of Ireland. When two Irish teams are competing against each other in a league match some Bulmers branding will be used, but the competition will always be referred to as the Magners League.
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