When a letter is eclipsed like this, the eclipsing letter replaces the sound of the original letter, so mbord is pronounced like bord, but with an m sound instead of a b sound. You don't have to try to pronounce both letters.
(When g is eclipsed by n, it's a bit more complicated, but as a general rule of thumb you just replace the sound of the eclipsed letter with the sound of the eclipsing letter).
Is there any common usage change in regional accents depending on formality of the speech or an adjustment (positive or negative) by the speaker to a listener?
As illustration my English pronunciation at work approaches a "standard midwestern accent" but my speech at home approaches an "Appalachian" pattern or "Ebonic" pattern depending upon which family member I'm talking to. It will also happen that if I'm out in public with a family member I tend to do the midwestern accent but if I want to freeze out a snoopy passerby I'll sometimes switch to the more obscure family dialect.
I'm asking this because I've read internet discussion which has compared certain Irish dialects to "Oxford English" and others to "Cockney". So I'm beginning to wonder how much Irish accents are intrinsic versus accommodative versus insular reactions.