No, you can omit the article with other nouns as well.
Féach:

AFAIR, some older grammars (e.g. O’Nolan) demand this kind of omission.

I'd say that means "What we want/would like is to know." (even possibly an emphatic form of "We want/would like to know."). It doesn't mean "That's what we want/would like to know". Do you see the difference?

What other possibilities did Dave have to offer?

I'll quote him in full, if you like -

You can say: is é rud gur mhaith linn 'fhios a bheith againn.

But normally wanting to know something implies finding it out (is é rud gur mhaith liom a dh'fháil amach).

Is é rud gur mhaith liom eólas a bheith agam air.

Note: you don't need "an rud" where the relative later defines it; rud is sufficient.

Is rud é seo gur mhaith linn 'fhios a bheith againn, agus sin é cúis 'n-a ndeinimíd ár ndícheall a dh'fháil amach ar shlí éigint.

Voilà
If it's more usual or natural to say - that's what we what to find out - in Irish in this context, rather than - that's what we want to know, so be it. It was just that I thought it rather strange that the equivalent phrase of - that's what we want to know - couldn't be expressed in Irish.

Haven't time to respond properly just now. from my previous post I repeat:do you see the difference?

Yes of course.

"What we want is to know (it)."



"What I want is to find it out."



"This is something that we want (we’d like) to know."

Go raibh maith agaibh