] Would you be so kind as to explain what MacWhatsit is? And why his/her interpretation is so highly regarded that it's up all over the internet?

] Also, does he/she give any insight into exactly what, and where, this Arci/Arcy is/was? (Normandy is big, y'know!)

Dr. Edward MacLysaght was the author of a book entitled 'IRISH FAMILIES Their names, Arms and Origins. First published in 1957, I was using the revised edition of 1972. Dr.MacLysaght was a member of the Royal Irish Academy, Chairman of the Irish Manuscripts Commission and Chief Herald of Ireland. He was also a very distinguished historian.
So there you go Caroline! Not even Lady Catherine could find fault with his pedigree.
However, I agree with you. Everything should be open to further questioning and I too would like to know the exact location of Arci, or was it Arques, as per Robert's posting. Not impossible. Flemmings did accompany the Normans at the time of the Invasion of England and they settled in Wales. The Norman invasion of ireland was Cambro-Norman rather than Anglo-Norman.

Also of note to the DeBourgh/Darcy connection is that the MacWilliams were a very important branch of the de Burghs.