So I'm currently doing an immersion program and tuilleadh and níos mó keep arising. My teacher says they're interchangeable, but he always says that one sounds more natural to him. Is there any rule on which one goes best with what type of words? Like why would tuilleadh airgid be more natural than níos mó airgid?

Regional, perhaps? Not sure...just throwing that out there.

If someone would care to explain, I, for one, have also been confused about when to use "tuilleadh" versus "a thuilleadh"

_________________
ЯГОН ТОҶИК НЕСТ ИНҶО???

I'm going to bump this back up, as I'm mightily curious about both questions.

Redwolf

I'd think:

(a) t(h)uilleadh is used in the pure sense of "something in addition"

a thuilleadh ("his addition") is used on its own as a quasi pronoun: Fuair mé a thuilleadh. = I got more.
And "a thuilleadh" is often used optionally with genitives: a thuilleadh eolais besides tuilleadh eolais
A thuilleadh eolais is definite, tuilleadh eolais is not. (This may perhaps affect the choice.)

But níos mó is used in a comparing sense (X > Y) with or without mentioning Y. Fuair mé níos mó (ná tusa).

In English both is "more", so níos mó is (perhaps) sometimes a kind of Béarlachas:
Ná habair níos mó. (obviously not: ... ná mise but aon fhocal amháin as do bhéal) replaces (more correct?) Ná habair a thuilleadh.

Of course any kind of addition can be thought to be comparing, e.g. níos mó eolais (ná mar a bhí agam roimhe sin)
Because of this níos mó can't be really wrong.
But a thuilleadh can: a thuilleadh agus a bhí agam roimhe sin is very dubious.

Okay so let's see if I got this right:

Ná habair a thuilleadh. means 'Don't say any more (about that)'
Ná habair tuilleadh. means 'Don't say any more (at all)'

So Máire is ranting about her husband, and so you say Ná habair a thuilleadh
Or you're out hunting deer and your stupid cousin keeps talking, so you say Ná habair tuilleadh

Then we're in a business meeting and the intern starts spilling the beans to a potential client so you say Ná habair a thuilleadh OR Ná habair tuilleadh OR Ná habair níos mó

Eh?

_________________
ЯГОН ТОҶИК НЕСТ ИНҶО???