The Celtic Literature Collective

Eiri mynyd
Harley Charter 75 C 384

Eiri mynyd guynt am bert
Cans creaudur nem am nert
Ai cyscy a una Gurnert

This is a very early version of the same, longer poem that appears in the late-14th century manuscript Llyfr Coch Hergest:

Eiry mynyd gỽynt am berth.
kanys creaỽdyr nef am nerth
ae kyſgu awna gỽrnerth

The charter dates to 1270, but the poem is dated pre-1150 by Kenneth Jackson, and Cowley, based on the handwriting (it's in an insular hand) and nature of the language (Old Welsh), it's assumed that the scribe was copying from an earlier lost manuscript, older than the 11th century.

Cowley, F.G. "An Old Welsh Englyn in Harley Charter 75 C 38-4. BBCS 25, 1974. pp 407-417.