This marks the group's second major sports sponsorship deal in the UK, having singed a four-and-half year agreement with Premiership football side Charlton Athletic in December last year.
The firm, which develops resorts alongside Spain's Mediterranean coast, also sponsor's Spanish La Liga premiership football team Valencia and the Menorca basketball team.
This latest deal with London Irish includes shirt sponsorship, as well as supporting its amateur club and community programmes.
According to a statement from London Irish, the deal is worth at least 拢2m but could increase depending on performance in the Guinness Premiership, Powergen Anglo Welsh Cup and Heineken Cup.
John Conlan, chairman of London Irish, which plays it sgames at Reading Football Club's Madejski Stadium, said: "This agreement is the largest sponsorship achieved in the club's history."
Fernando Gallego, president of Grupo Llanera, said: "Developing awareness of our brand and positioning it among key target audiences in the UK is essential to the future growth of our company."
Grupo Llanera replaces Guinness as London Irish's lead sponsor, but the drinks giant will continue its association with the club as its official beer partner.
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