From Vulgar Latin illi, which sounded il in Old French, to which a plural -s was added. While il was the nominative form from late Latin nom. *illi of ille, els, eus was the oblique form ("them") that had evolved from late Latin *illos, it is the ancestor of modern French eux.
In Old French, "they", being a nominative, was il from late Latin *illi, thus it didn't have the final -s, thus it was il used both for "he" and "they". The -s was added at the end of the 13th century in some regions, at the time the declension system of Old French started to collapse. As a consequence, some oïl languages in France have retained the original Old French il-form, and in some other regions, the ils-form supplanted the older one. Some dialects have even retained both forms depending on the locals.
See cognates in regional languages in France : Angevin is, Bourbonnais-Berrichon ils, Bourguignon âs, Champenois is, Franc-Comtois és, Gallo i and iz, Lorrain is, Norman i and is, Orléanais is, Picard is and i, Poitevin-Saintongeais és, Tourangeau is, Franco-Provençal ils, Occitan els (Gasconian eus), Catalan ells, Corsican egli.
|Singular||First||--||je, j'||me, m'||--||--||moi|
|Third||Masculine||il||se, s'||le, l'||lui||y||en||lui, soi|
|Feminine||elle||la, l'||elle, soi|
|Third||Masculine||ils||se, s'||les||leur||y||en||eux, soi|