Definition of "byrnie"

Unferth stood beside him, his huge arms folded on his byrnie.

1972, John Gardner, Grendel, André Deutsch, page 97

The mail-coat, or byrnie, was made of iron links that probably were cut out of sheet metal with a die, or from flat hammered wire cut into short lengths.

1992, Calvin B Kendall, Voyage to the Other World, University of Minnesota, page 19