Eiri mynyd
Harley Charter 75 C 384
Eiri mynyd guynt am bert
Cans creaudur nem am nert
Ai cyscy a una Gurnert
NOTES
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.
SOURCE
Cowley, F.G. "An Old Welsh Englyn in Harley Charter 75 C 38-4. BBCS 25, 1974. pp 407-417.