According to ISTAT Ilenia has been decreasing in popularity since 1999. In that year were born in Italy 787 Ilenia while in 2018 only 196 (the lowest number since 1999 -national statistics-). In 2018 Ilenia was #150 most popular name for feminine newborns.
Ilenia could be an Italian form or elaboration of different names:1 Ileana / Ilona (Romanian forms of Elena) 2 Jelena (Croatian and Serbian form of Elena) and Yelena (Russian) 3 Ilana (אִילָנָה, feminine of Ilan, Hebrew name meaning "tree")