How did English end up with the adjective fiery (instead of *firy) from fire, but miry from mire and wiry from wire?
Are there any other words where the noun is -ire and the adjective is -iery?
The Online Etymology Dictionary explains the unusual spelling:
late 13c., from Middle English fier “fire” (see fire (n.)) + -y (2). The spelling is a relic of one of the attempts to render Old English “y” in fyr in a changing system of vowel sounds.
Words like miry (late 14c.) and wiry (1580s) have later origins and different etymology, so they don’t have the same influence on their spelling.