This is an extract from P.W. Joyce's Irish Names of Places, Volume III, and is a fascinating insight into the meaning of the placename Navan, which may link it with the Milesian invasion, which also has strong associations with Drogheda, about 14 miles downstream.
The name of Navan, in Meath, has long exercised Irish etymologists—including even O'Donovan. This greatest of all Irish topographers identified it at the time he was employed on the Ordnance Survey with Nuachongbhail, which is often mentioned by the Annalists; or perhaps it would be more correct to say that he showed beyond doubt that Nuachongbhail stood where Navan now stands. Nuachongbhail signifies new habitation, from nua, new; and congbhail, a habitation. This long name would be sounded Noo-hong-val; and elsewhere in Ireland it has been softened down to Noughaval and Nohoval. L is often changed to n in Irish names, and if we admit that this has taken place here, and that the middle h sound has been omitted (which it often is, as we see in Drogheda for Droghed-aha, Drumlane for Drumlahan, &c), we shall have the form Novan; and we know that in some old documents, written in English, the place is called Novane.
But another very different, and indeed a far more interesting origin for the name suggests itself. We are told in several of our most ancient legendary records, that Heremon son of Miled or Milesius, while still living in Spain, before the Milesian expedition to Ireland, married a lady named Odhbha [Ova] who became the mother of three of his children. After a time he put her away and married Tea, from whom in after time, according to the legendary etymology, Tea-mur or Tara derived its name. When Heremon came to Ireland, Odhbha followed him and her children, and soon after her arrival died of grief on account of her repudiation by her husband. Her three children raised a mound to her memory, which was called Odhbha after her; and from this again was named the territory of Odhbha which lay round Navan, and which in after ages was known as the territory of the O'Heas.
This mound we know was (and is still) near the place on which Navan now stands; and like all sepulchral mounds, it must have contained an artificial cave in which the remains were deposited. We know that the present colloquial Irish name of Navan is an uaimh, "the cave": this name is still remembered by the old people, and we find it also in some of our more modern Irish annals. We may fairly conclude that the cave here meant is that in which Queen Odhbha has rested from her sorrows for three thousand years; and it may be suspected that uaimh, though a natural name under the circumstances, is a corruption from Odhbha, as both have nearly the same sound; in fact the modern pronunciation varies between an Uaimh and an Odhbha.
Another element of difficulty is the fact that in the Annals of Lough Key the place is called An Umamá—"The Umamá"—which seems to show that the old writer was as much puzzled about the name as we are, and wrote it down honestly as best he could, without attempting to twist it into an intelligible word, as many modern writers would do without hesitation. This form Umamá is probably evolved from the old form Odhbha—at least I shall regard it so.
Now, from which of these three words, Nuachongbhail, Odhbha, or An Uaimh, is the name of Navan derived; for it is certainly derived from one or another of the three? The first n of Navan (as representing an uaimh)is the Irish article an, contracted to n, as it usually is; and this is still remembered, even by the English-speaking people, for Navan has been and is still often called The Navan. But this fact might apply to any one of the three derivations. In the case of Navan coming from Nuachongbhail, the first n of this Irish name was mistaken for the article; just as in the case of Oughaval in Sligo, Mayo, and Queen's County, in which the initial n has been dropped by the people, who mistook it for the article, the proper name being Noughaval, i.e. Nuachongbhail; and as to Odhbha and Uaimh, the article is there to the present day annexed to both. The presence of the last n of Navan is quite compatible with the derivation from either Odhbha or An Uaimh, for it is the termination of an oblique form, and as a matter of fact uaimh is often written and pronounced uamhainn, as in the case of the name of the village of Ovens, west of Cork city, which is really Uamhainn, i.e., caves, from the great limestone caves near the village, and either 'n-Odhbhan or 'n-Uamhainn would sound almost exactly the same as the old English name, Novane.
The change from Nuachongbhail to Novane looks too violent, though possible, and I am disposed to believe that Queen Odhbha's name still lives in the name "Navan." The people having lost all tradition of Heremon's repudiated queen, and not understanding what Odhbha meant, mistook it for Uaimh, which has nearly the same sound, and which was quite applicable, as the cave was there before their eyes, so they prefixed the article and used Uamhainn (as elsewhere) for Uaimh, the whole Irish name, n-Uamhainn (pronounced Noovan), being Anglicized to Novane, which ultimately settled down to Navan. But this is by no means certain, and until we discover more decided authorities the name will continue doubtful and tantalizing.