ion-, ionn-, a frefix of like force as Latin in-, used especially before medials, liquids, and s ( ionn- only before s), Irish in-, ion-, inn-, ionn- (before s), Old Irish in-; it is the Gadelic prep. in, ind, now an, ann, in (q.v.), used as a prefix.
Etymological dictionary of the Gaelic language.
Alexander Gairm Publications.
1982.