Translation:For the most part.
It's (probably) the preposition do (could be de), which lenites the following noun, and then becomes d' before vowel sounds.
I never hear this word pronounced and its driving me crazy... would it be like doerver?
formhór is "majority". d'fhormhor on it's own is just "for the most part", an idiomatic expression in Irish, just as it is in English. It can also occur in a sentence where it would mean "for most" or "for the majority".
*that resolution is optimal for most drawings" - is é an taifeach sin is fearr d'fhormhór na léaráidí
"construction was the main driver of the economy" - an tógáil a bhí taobh thiar d'fhormhór dul chun cinn na heacnamaíochta
I know. I'm getting aggravated with how many things have no recording to accompany them.
I never understood how you get four words out of one, or technically two with the preface D' Can someone remind me?
Just observed: "mostly" (as in one word;-) is accepted as correct answer.
There's no one-to-one correspondence with the Irish phrase in this case; "for the most part" is a stock phrase in English, and doesn't even make much sense if you think about it. Just weird English.