Apologies if this has been asked before. I know both are used, but are they completely interchangeable or does it depend on the dialect? I always used as Gaeilge until recently, but have found myself saying i nGaeilge from time to time.

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The traditional usage and common enough in Munster today is as for speaking and i for writing.

Labhair as Gaelainn é Say it in Irish.
Cur síos air i nGaelainn Write an account of it in Irish.

I think in other dialects they use trí for speaking (anybody?).

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

In dialect books I've seen 'i nGaeilg' and 'i nGaolainn' etc.

Using the genitive form for the common is a Conemara thing AFIK and I was told once by someone the use of 'as' while not wrong was less frequent and done as it involved no inflection and so was easier for learners

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I've wondered about this as well. I've certainly heard 'trí Ghaeilge' before on TG4 and in Conamara but tbh I never knew what it meant exactly. I thought perhaps it means 'through [the medium of] Irish'?

Hopefully someone in the know can explain the differences.

Its the genitive form mostly in Mayo as well apart from Acaill, and from recordings I listened to from Clare they said Gaeilge in most places as well.

To be honest I've only heard learners say ''as Gaeilge'' - I prefer ''i nGaeilge''.

for written and spoken Irish?

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For spoken I've heard it mostly but if you say ''i nGaeilge'' then thats what you write.

Really, that's quite different from Munster then. What dialects is i nGaeilge more common in? It's useful to know this stuff!

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

Interesting - I woudn't know since I've only been to Corca Dhuibhne twice.

I speak Connemara Irish and ''i nGaeilge'' is more common, and ''i nGaeilg'' seems to be more common in Donegal as well.