It may be interesting to some readers of "NOTES AND QUERIES" to know that exactly the same thing has happened in Ireland, and that the recognised Anglicised forms of several proper names, now stereotyped, are a combination of the definite article an, of the Gaelic or Irish language, with the name of the place.
For instance, Nenagh in the co. Tipperary is properly Aonach [pron. eenagh], but generally spoken of by the people with the definite article an Aonach, the Aonach, i.e. fair, place of a fair or assembly; and hence by the English made Nenagh.
So also the river Ainge [pron. nearly as Anny] is usually called an Ainge, the Ainge; and therefore is now Nanny, the Nanny, or Nanny water, in the co. Meath.
In like manner, the island Aondruim in Loch Cuan, on which stood once a celebrated monastery, is in Irish always called an Aondruim, the Aondruim, and is now Nandrum or Nantrim Island.
The town of Newry is another instance. It has its name from an ancient yew tree [in Irish Iubhair, pron. nearly as the word your] which stood near it, and was said to have been planted by St. Patrick. Hence the town is always called an Iubhair, the yew tree; which, by incorporating the article, has been Anglicised Newry.
The river Nore in Ossory, is properly an Eoir, the Eoir [pron. Ore].
So also the Navan fort near Armagh, is an Eamhain, the Eamhain [pron. nearly as Avan].
I might fill a page with other instances, but I shall only mention another similar corruption in proper names, where after dropping the Mac the c is retained, in cases where the patronymic begins with a vowel. Thus the descendants of the Danish family of Ottar became Mac Ottar, and are now Cotter. So Mac Etigan became Gettigan; Mac Eeoghegan, Geoghegan; the c being further transmuted into g. And hundreds of similar instances could be given.
It may also be observed that the English very generally caught the genitive, or oblique case, of the Irish proper names, and from it formed the name which is now in use amongst the English speaking population. Thus they heard the Irish speak of the isles Araun, i.e. the isles of Ara, for Araun is the genitive; and hence they are now the Aran Isles. So also the ford Trim or Druim, in Irish Ath-Druim (the ford of the long low hill, vadum Dorsi), where Druim [pron. nearly Trim] is the genitive of Drom or Drum, a long low hill, a back.
The names given to Ireland by medieval writers, after the ancient name of Scotia had been transferred to Alban (which, by the way, is itself a genitive, from Alba), afford instances of the same thing.