The "sheep letter" contains laws for sheep farmers which were in force
in the Faroe Islands during the middle ages. I'm not sure if there's
an English tanslation anywhere online, but there are images of the
text as it appears in a 14th century manuscript (the original goes
back to 1298). That's the link Patricia found:
'seyðarbræfið' gets lots of hits on Google, so there might be some
more detailed information out there. It even came up with something
from the norse_course archives.