The Roman Map of Britain Nagnatia? Achill Island? (County Mayo, Ireland)

Magantia [/Nagnatia] var. Magancia  (R&C 290) next

    The line preceding the entry Magantia, Item ad aliam partem dicitur insula would allow that the cosmographer left the previous area between Britain and Ireland, and was reporting islands from another part of the map. These would be of comparable latitude, but with a shift to the west. I would remind the reader that this section is simply described as 'Iterum in ipso oceano occidentali ponuntur diverse insule', i.e. islands situated in the western ocean.
    Ptolemy records the Nagnatae (var. Magnatae), an Irish tribe, and their coastal city Nagnata (var. Magnata). Nagnata is on the west coast with the Libnius river to the south and the Ravius river (Erne) to the north. Patrizia de Bernardo Stempel¹ represents the Ravius in the northeast corner of Clew Bay, suggesting that perhaps Ravius is the Srahmore river that rises on the slopes of the Nephin Beg Mountains. The Srahmore, joined by the Glenamong River, enters Lough Feeagh then Furnace Lough to Clew Bay. *Nagnatia (insula) would represent a name formed from a tribal name, much as Epidium promontorium or Novantarum peninsula.
    The island of Achill seems to be rightly positioned, and is the largest of Ireland's islands.
    I'd originally thought Magnatia the better form, but manuscript authority in the Geography is in favor of Nagnatia.

1. p 112 Ptolemy's Celtic Italy and Ireland: a Linguistic Analysis in Ptolemy: Towards a linguistic atlas of the earliest Celtic place-names of Europe (2000)