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.
Ils is used for groups with only masculine nouns or mixed groups with at least one masculine noun. If the group referred to contains only feminine nouns, elles is used instead.
|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|
- “ils” in le Trésor de la langue française informatisé (The Digitized Treasury of the French Language).