Translation:For the most part.
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