I know that, in written Irish, g is eclipsed by n, but is the actual sound that eclipses g 'ng'? For example, would 'i nGaillimh' be pronounced 'i ng-aillimh' or 'i n-aillimh'? I just want to clarify this.

Yes, it's the velar nasal. Same as in English 'sing'.

Yep. Sometimes a hard concept for English speakers to get right away (I've had students ask me "how can you put "ng" at the beginning of a word?"). When I teach it, I usually use "i ngrá" as an example. I have students say "sing raw" a couple of times, and then drop the "s" off of "sing." That usually helps them to get it.

Redwolf

Thanks for the clarification. I was getting mixed messages about it. I was taught that the letter that eclipsing the other one is the one that is pronounced (e.g. in 'nDoire' the D is effectively dropped) but I suppose it would look a bit unusual to write 'ng' at the start of a word because that would result in a double g, e.g. i ngGaillimh', which, as far as I know, isn't seen in Irish so it would look a bit strange. I was wondering because, in the other cases of eclipsis, the consonants just turn into a more 'voiced' version of themselves but they are voiced from the same part of the mouth, so it would make more sense for g to turn into ng than into n.

That's exactly what's happening. Voiceless consonants become voiced, and voiced ones become nasal, all keeping the manner and place of articulation. You can find some interesting information on the origin of the initial mutations on this Gramadach na Gaeilge page

Go raibh maith agat to both to you for bringing this up! This has totally cleared up my newbie confusion around eclipsis.

That sounds like a very straightforward way to explain it.

I was recently trying to write a description using English sounds how to say 'i gceann' and couldn't come up with anything close. That is without mentioning the 'ch' sound in something like 'mo chos'. We need a 'tearing my hair out' smilie.

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"ig yonn" ?

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Thanks for that. That is definitely better than what we had. We had the i on its own, but couldn't figure gcionn. What you have is much better.

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Or even "ig yawn".

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).