Sometimes I see "i nGaeilge" with the English meaning "in Irish", but sometimes "as Gaeilge". Is there any difference?

[Cleachtadh: Uaireanta feicim "i nGaeilge" leis an chiall Béarla "in Irish", ach uaireanta "as Gaeilge". An bhfuil aon difríocht ann?]

Go raibh maith agat as do chuidiú!

It means the same, just a different perspective.
I'd use as for verbs of speeking ("Abair as Gaeilge é!") but i for writing ("Scríobh as Gaeilge é" doesn't make much sense for me)

It's depends on dialect.
In Conamara we say "i nGaeilge" for both speaking and writing.

Learners, or people who have no interest in Irish,find "as Gaeilge" easier, so they've latched on to that. It's what you see in all the English language media.

I nDún na nGall fosta, scríobhtar agus labhraítear i nGaeilge.

From the corkirish blog dictionary (and a short post about it), I also believe Peadar Ua Laoghaire (and perhaps other Cork authors?) also tended to use i nGaelainn generally and a Gaelainn (or as Gaelainn) only in the context of speaking.

There’s been also an older thread about it, i nGaeilge / as Gaeilge here – people suggested back then that as Gaelainn is typical for Munster (and only for speaking), other dialects using i nGaeilg, i nGaeilge; and there’s also trí Ghaeilge in use for through the medium of Irish.

BTW, according to Ó Sé’s Gaeilge Chorca Dhuibhne, as typically causes lenition in Dingle Irish. But still not with languages, he gives examples bhí a ainm as Gaelainn and dh’fhiafraigh sé dhóibh as Béarla; no example of i nGaelainn, but there’s nach mó constaic atá insa Ghaelainn? but I don’t think that’s the same sense of in Irish we are speaking about here (but there is no context provided).

Thanks everybody. I prefer to learn Standard Irish or Ulster Irish, so "i nGaeilge” for me.

[cleachtadh: Go raibh maith agaibh a chách. Is fearr liom Gaeilge Chaighdeánach nó Gaeilge Uladh a fhoghlaim, mar sin “i nGaeilge” dom.]

"as Gaeilge" is a blend of Munster and standard Irish, only learners use it.

Connachta: i nGaeilge
Donegal: i nGaeilg (pronounced as if "i nGaeilic")
Munster: i nGaelainn & as Gaelainn (cf preceding answers).

I think "as Gaeilge" is being taught in schools for 1 reason only: "as" doesn't trigger any mutation so it's simpler to beginners... But what's the point teaching something that no native speaker ever says?...

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

It is simply Standard Irish ... which of course is a blend of dialects spoken by learners.

Slightly off topic, but is there any relationship between using the Munster "Gaelainn" and its variants as nominative, and "Éirinn" in the phrase "Éirinn go bragh" as a nominative? There was a time when some people said using "Éirinn" was incorrect (the "Éire" should be used) but I've since learned the "Éirinn" may be a nominative form in some areas, i.e. Munster.

Not a specialist on Irish dialectology, but I believe Éirinn is the spoken form everywhere in the Gaeltacht just as Albain (and not *Alba, the old nominative) is for Scotland. Éire is just an archaic form that sticks (and is used in the Caighdeán) because, I think, of its use on banners, coins, some phrases, that make it resistant to be entirely lost; on the other hand the name of the other country, Alba, disappeared entirely and only dative Albain remained (funnily, but maybe not that surprising, in Scotland it’s the reverse, Ireland is always Èirinn while Scotland is Alba in nom. and Albainn in dat. – but there I think Alba is the dominant form in spoken language and dative rarely used).

Nothing to do with the -inn in Gaelainn though, except that it also comes from archaic dative – it is just a Munster form of older Gaoidhilg, Gaedhilg (not sure how exactly it evolved, but the final -ilg seems to have changed to -lig and then to -ling), the dative of Gaoidhealg, Gaedhealg (nominative not used anywhere today afaik, though Wikipedia gives Ulster Gaedhlag which looks like coming from it). The modernized spellings of the unchanged dative such as Gaeilg, Gaeilig, or Gaeilic, is afaik used in Ulster as the name of the language. The standard and Connacht Gaeilge is modernized spelling of the genitive Gaoidhilge, Gaedhilge.