The brothers designed Bord na Mona's 2014 main avenue garden and 2013 artisan garden. Bord na Mona UK consumer head Steve Harper said the garden would "surprise everyone".
Meanwhile, M&G, Laurent Perrier and M&G will sponsor show gardens.
Andy Sturgeon is expected to return to chase a seventh Chelsea gold. Hugo Bugg could return for a second, while former RHS shows head Stephen Bennett's son Luke Bennett is planning a butterfly feature on the triangle site. Designs submission deadline is 11 August for the 19-25 May 2015 show.
Plans indicate space for 19 show gardens.