In response to my own original message, and adding to Frances's note on the 1367 Kilkenny Statute, my recollection of reading about a Manx Law must have refered to an Irish Law and was probably Kneen's reference to Statute of 5 Edward IV (1465) in his Introduction to Personal Names of the Isle of Man.

If anyone is interested, further info on the 1465 Law can be found on the WWW, for example, the following extract from AN ESSAY ON THE ORIGIN AND IMPORT OF FAMILY NAMES by William Arthur:

"In the year 1465, in the reign of Edward the Fourth, it was enacted by statute, that every Irishman dwelling within the English pale, then comprising the counties of Dublin, Meath, Lowth, and Kildare, in Ireland, should take an English surname.

"At the request of the Commons, it is ordeyned and established by authority of said Parliament, that every Irishman that dwells betwixt or among Englishmen, in the county Dublin, Myeth, Uriell, and Kildare, shall goe like to one Englishman in apparel, and shaveing off his beard above the mouth, and shall be within one year sworn the liege man of the king, in the hands of the lieutenant, or deputy, or such as he will assigne to receive this oath for the multitude that is to be sworne, and shall take to him an English surname of one towne, as Sutton, Chester, Trym, Skyrne, Corke, Kinsale ; or colour, as White, Black, Brown ; or art or science, as Smith, or Carpenter ; or office, as Cook, Butler; and that he and his issue shall use this name under payne of forfeyting of his goods yearly till the premises be done, to be levied two times by the yeare to the king's warres, according to the discretion of the lieutenant of the king or his deputy."-5 Edward IV., cap. 3.

In obedience to this law, Harris, in his additions to Ware, remarks that the Shanachs took the name of Fox, the McGabhans or McGowans, that of Smith, and the Geals the name of White. In consequence of this statute of Edward, many Irish families were induced to translate or change their names into English."
Ref: www.searchforancestors.com/surnames/origin/essay.html

Perhaps the situation in Ireland had an influence on the Manx surnaming patterns as the Tudors and their allies took up positions of power in the Island, and possibly leading to the formation of the C, K and Q Manx surnames from Mac**** names in the 1515-1570 period.

Maybe someone can find a similar Manx or Irish Law for the 1515-1570 period which may have had direct influence?