This form of the headword (present indicative active first person singular) appears only in the Suda, although a participial form does occur in a text ascribed to Cyril of Alexandria: PG 77.1229.55. The non-compounded verb ἱλαρύνω ("I gladden, I brighten") is found often in in the Septuagint and in Christian writers.
[1] Adler cites the Syntacticum Gudianum for this assertion, but it is not true as far as we can tell from the single attestation of a form of the headword that we possess, and from the several attestations of the uncompounded verb; both are usually construed with the accusative rather than the dative (and compounds verbs in κατα- do not generally take the dative; see Smyth 1545 and 1690.3). The Lexicon Syntacticum of Codex Laurentianus 59.16 ascribes the dative to a similar word καθιλαρεύομαι , an assertion which is likewise unsupportable in the surviving attestations of the word (cf. kappa 109).