Why is it fear and not fhear?
Lenition is used with feminine nouns, but not with masculine nouns.
If you don't know, which noun is what, you can look it up:
Is eclipsis used with masculine only?
Eclipsis is not used to mark gender in Irish.
Because of the male and femine thing
Small mnemonic: when you're presented a list of arguments for something, BUT... and then a bunch of arguments against it, you'd probably go "ACH, of course".