The authors agree that the surname derives from the Latin personal name Hilarius, itself related to the adjective hilaris, meaning "cheerful, jovial". E. De Felice notes that its spread as a given name was consolidated by the cult of Saint Hilary of Poitiers, a 4th-century bishop, and that its fixing as a surname occurs in central and southern Italy, where it appears sporadically.

E. Rossoni likewise links the surname to the Latin cognomen Hilarius, a name borne among others by Pope Saint Hilarius (461–468). He documents an early example of its use as a personal identifier in 1022, in a deed from Benevento which mentions a certain Ilario di Telese. According to Rossoni, the surname Ilari shows specific areas of presence in the southern Marche, in Lazio and in the Trapani area. The variant Ilario, more infrequent, has cores in the Naples area and in lower Lazio, while Ilarini appears to have died out.