Had to look it up (curioisity killed the cat;-): This is one of the exceptions, see GnaG use of the copula:
The use of the copula is limited in Irish to the following, specific, yet frequently occurring uses:
•adjectives of subjective assessment and of equivalence. e.g.: Is ionann iad = They are the same. Is maith sin! = That is good!
I will no doubt stand corrected, if this is wrong interpretation.
I'd also like to know this. I'm still confused as to whether this sentence is strictly used for a lot of physical objects between two people or what figurative uses it has (in non bastardised Irish)
Does this mean in terms of "age", that is, that one is far older than the other, or in terms of "emotions", that is, they fell out and now they have a lot of issues with each other?
More the former, but it can apply to any characteristic: height, weight, personality etc.