Eclipsis and pronunciation
This orthography is still messing with my head, so I just wanted to write this down before heading back down a rabbit hole of IPA charts because it made eclipsis a bit more sensical for me.
Lightly place your hand on your throat. Now say 'fuh.' You shouldn't really feel anything, maybe your Adam's apple moving. Now say 'vuh.' Notice that your mouth makes the same motion, but now you can feel your larynx vibrate. The 'fuh' sound is voiceless, and the 'vuh' sound (spelled 'bh' in Irish) is voiced.
Eclipsis takes (most) voiceless consonants and makes them voiced. 'Cailín' becomes 'gcailín,' 'portán' becomes 'bportán,' and 'turtar' becomes 'dturtar' (the second consonant in the eclipsed word is silent)!
"But what about b and d," you say? Those two voiced consonants become nasal ones ('m' and 'n,' respectively)! Say 'doe' (you don't have to do the hand thing anymore), and then say 'no.' Your mouth makes roughly the same motion again, but they feel different. This is because when you say 'nuh,' most of the air is escaping through your nose rather than your mouth. Compare bode/mode and doze/nose in English. Likewise, 'doras,' meaning door, becomes 'ndoras,' which pronounced like 'noras.' 'Bainne' also becomes 'mbainne,' pronounced with a silent 'b.' You can see the reverse of this effect when you have a cold ('I have a ruddy doze...' takes the nasal 'n' and replaces it with a similar but non-nasal 'd' because your doze is all stuffed up).
'G' is similar to 'b' and 'd,' but see how your tongue is a bit further back in your mouth when you say it, compared to 'd'? When you nasalize 'guh,' it ends up sounding like 'nguh.' I can't think of any base pairs in English for g/ng sounds, but you can see the difference between 'log' and 'long.' (Also, 'ng' is not 'n'. Say 'lawn' and then say 'long.' Feel where your tongue is at the end of both words.)
Hope the helps someone out there. I'm learning this as I go, so if I'm wrong about anything, let me know.
The fourth paragraph confused me a bit. Could you maybe give some example words for the nasal sounds?
Irish, I suppose. Like how you mentioned 'cailín' becomes 'gcailín' :]
Sure thing! Bainne (milk) becomes mbainne, which is pronounced with a silent 'b.' It ends up sounding sort of like 'manya.' Team Irish's lesson notes have a neat little table with one example for each eclips-able starting letter that you might find helpful.
I've tried to clear up the original post and add some more examples, let me know if it helps.
Go raibh maith agat! That clears that up. I have seen the tables in the notes, I just need to go over them a few more times and commit them to memory.
Yes, eclipses turns voiceless consonants into their voiced variants and voiced consonants into their nasal variants.