A fair number of the songs in Geas are adapted from old English ballads, as they tend to fit the general theme (although someone really should change all references to "Irish," "Ireland," "English," "England," or "London"!) even if not genuinely medieval.
Anyway, I came upon a great archive of Early Modern English ballads, starting around 1500, some of which are accompanied by scholarly recordings.
http://emc.english.ucsb.edu/ballad_project/index.asp
English Ballads
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