I don't see that.
The "is" is already included; the full copular sentence would be
(Is é) an rud is tabhachtaí an aithne.. - The thing that is the most important is the knowledge..
By placing "ná" after the adjective you turn it into a comparative as far as I can see, or at least that's the most general use in a sentence like this.
Here's a list of some of the uses of "ná" and I can't see which of these would apply, so I'm hoping fiairefeada will explain it to me, maybe it's a dialectical thing.
Ní raibh aon duine beag
ná mór ann - There wasn't anyone neither big nor small there
ná = nor
An fear is mó
ná mise - The man that is bigger than me.
ná = than
Cé a casfaí orm
ná an fear mór é féin - Who should I meet but the big man himself.
ná = but
Níl aon duine sa bhaile
ná go bhfuil a fhios aige cad is ainm dó - There isn't anyone in the town that he doesn't know their name.
ná go = but that
And of course the famous
Is é ainm a bhí air
ná Séadna - The name that he had was Séadna.
ná = (pleonastic) it has function rather than actual meaning.
Anyway maybe there's more but I don't know it. Hope ff comes on soon, because this is quite interesting. (Yes I don't get out much)
