I just came across this interesting article put up by David Webb:

Is agus Tá

Interesting explanation!

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Bí cinnte de go nglacfaidh triúr le gach aistriúchán a thabharfar.
Be sure to get three in agreement with a translation given.

Well, as far as I know, it's wrong. We now know that in Old Irish, in some cases you could use "atá" when it would be wrong nowadays, and you could use "is" when it would be wrong or strange nowadays. Maybe the author of that article thought about Classical Irish... anyway at that time, scholars didn't know Old Irish as we know it now.
You one need to open Thurneysen's grammar of Old Irish, to see examples with "atá" and "is" used in a way that would be wrong in Modern Irish...

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