Building on Bron's trusty Welsh dictionary, I'll try to add a bit, hoping that Sian or Tegwyn will show up and bail us out!

"Werin daear" is probably "People of the earth," and I wonder if "o'ch" has the sense more of "from out of" the heavy chains....

"Cerddwch" is probably the command form, so "walk to the hills"

Likewise, "Clywch" in the command form would be "hear the multitude on the steep hills"

Humbly acknowledging the full limitations of my Welsh abilities!..... Margaret