The Roman Map of Britain Adron or Agron River Ayr

Adron (R&C 254) next

    In this situation both IE -dr- and -gr- yield -ir-, thus adron and agron yield W. aeron. See LHEB pp 429-31. The scribal error of d for g is possible, but not certain. In any case, the next entry is the River Cree
    Afon Aeron is a Welsh river (Cardiganshire), that according to Sir E. Anwyl is based on the name of the goddess Agrona 'Slayer' from ag-ro-s*. On R&C p22, Ifor Williams has Agron 'Slaughterer'. Sir J. Morris-Jones equated the Aeron of the Book of Taliesin with the Scottish Ayr. R&C say only that it is not the Berwickshire Adders (The Whiteadder is Withedre 1165-1214.). 
    The Ayr is Ar a.1177, Are ca. 1197, Air ca.1230. Johnston (1934) offers ON eyri, 'tongue of land, gravelly bank.'

*ag-ro-s 'driving, rushing' Pokorny Indogermanisches Etymologisches Woerterbuch p 11.
Ir. âr `defeat' (*agron), W. aer `battle, fight' (*agrâ), actually `baiting', OCorn. hair glossed L. clades 'destruction'.

Book of Taliesin / Llyfr Taliesin
    view online manuscript at the National Library of Wales [aeron folio 14. verso.]
    .pdf transcription by Candon A. McLean