Would appreciate guidance on when to use i v e for 'it:
I tried grànda but it was not accepted - what is the difference between the two?
uabhasach is extremely? uabhasach math = very good i thought
Not quite. Uabhasach means horrible. Uabhas on its own means horror or terror. Uabhasach math is often translated into English as very good, however it would be more accurate to say "horribly good".
It can work as an intensifier with that meaning in that context, but literally it means "terrible".
Why is eagalach not accepted?
Because 'eagalach' is scary, whereas 'uabhasach' is terrible.
cianail not acceptable?
cianail means 'lamentable' or 'sad' more. It only translates to 'terribly' when used as an intensifier, not on its own.
does this not mean he will be terrible?
It means both.
My dictionary says eagalach means terrible
It's terrible in the sense of scary. Eagal is fear. Eagalach is scary.