Free and Open Source Software (FOSS) has gone mainstream: the amount of FOSS libraries, components and programs available has grown at an amazing pace, to the point that we can find them used in practically every existing software project developed nowadays. It's a great success, but with this great success come great challenges: the rapid growth of the FOSS code base and its decentralised development model pose new, difficult problems that need to be solved in order to master the increasing complexity that we are facing. Irill is a unique initiative established four years ago to tackle these problems, by Inria, University Paris Diderot and University Pierre et Marie Curie, with the precious support of the Île de France Region. Irill connects top computer science researchers and FOSS developer communities to work together on innovative theory and tools that enable better FOSS software to be built. In this talk, we will provide some highlights of what has been accomplished in the first four years of existence of Irill, and give a glimpse of what is in store for the future.