"The man has shirts."
Translation:Tá léinte ag an bhfear.
Think of this as an organic thing. In general, eclipsed/lenited sounds are made in the same part of the mouth as the original sound (this is not universally true - e.g. d to dh is different).
You have a pattern for eclipsis:
Unvoiced (e.g. p) to voiced (e.g. b) to nasal (e.g. m)
In this, f (an unvoiced bilabial fricative) is eclipsed to a v sound, written bh (a voiced bilabial fricative). There are both slender and broad versions of this. The broad version can be pronounced more like a w sound outside Munster. That sound makes sense if you round your lips and blow out whistle-like. Some people also insist on using the English labiodental fricative sounds here.
Plosive (e.g. p) -> fricative (e.g. f) -> a gap (in the case of fh)
Either a broad or slender f, if you open your mouth more before blowing, you create no sound.
Hold air behind lips, build up pressure and release it. You get a p if you don't voice it and b if you do. Now do the same but release the air in a controlled fashion through your lips. You get an Irish f (ph) or an Irish v (bh).
bh is not lenited but f is, making the silent 'gap' sound mentioned above.
eclipsis and lenition are mutually exclusive - it's one or the other, never both.
Words that start with f are eclipsed by bh.
To be fair, from using the Duolingo app myself to start the course, I didn't learn until recently that the web version is more comprehensive and that there were even notes sections at all. Am really appreciating those who have taken the time to chime in on the threads with links, more directions, etc.!