found what is possibly your source - Irish the 3rd article of statute of Kilkenny 1367 (according to most it had little or no effect
(see http://www.ucc.ie/celt/published/T300001-001/ for text)

III. Also, it is ordained and established, that every Englishman do use the
English language, and be named by an English name, leaving off entirely the manner of naming used by the Irish; and that every Englishman use the English custom, fashion, mode of riding and apparel, according to his estate; and if any English, or Irish living amongst the English, use the Irish language amongst themselves, contrary to the ordinance, and therof be attainted, his lands and tenements, if he have any, shall be seized into the hands of his immediate lord,