chain mail vs byrnie

chain mail and byrnie both are nouns.

Word	Noun	Adjective	Verb	Adverb
chain mail	Yes	No	No	No
byrnie	Yes	No	No	No

As nouns, byrnie is a hyponym of chain mail; that is, byrnie is a word with a more specific, narrower meaning than chain mail:

chain mail: (Middle Ages) flexible armor made of interlinked metal rings
byrnie: a long (usually sleeveless) tunic of chain mail formerly worn as defensive armor

Other hyponyms of chain mail include brigandine, habergeon, hauberk, gusset, voider.

chain mail (noun)	byrnie (noun)
(Middle Ages) flexible armor made of interlinked metal rings	a long (usually sleeveless) tunic of chain mail formerly worn as defensive armor