quote:you look at a declension table
Yes, but first you have to know word's declension. There is almost no way to say declension beforehand. That was my point. And even with declensions there are too many exceptions like endings -anna and -acha which spread far beyond original 5th declension.
quote:? (tempted to say ‘tón’ as I’ve never heard arses!)
It is "tóinte" - 3rd declension but irregular plural
quote:talaimh
This is irregular noun, so plural is "tailte"
quote:duine - daoine
so, looking at which pattern did you do this plural? or you just happen to know it by heart? :)))
quote:lán
This is adjective "full". Don't understand what kind of plural you make
quote:ceist - ceisteanna
This is second declension. How is anyone supposed to know it is not "ceiste"?
quote:bóthair
This should be "bóithre"
etc.
I don't see your quick-n-simple method giving good result so far