"Is maith sin" literally "Thats good".
You will not find it's derivation in English dictionarys as there has to be an example, in the written form, in order for the compilers to have a proven point of reference. Try sending this to the OED and you may see it being credited. It is most certainly not of an unknown derivation.
" 'S an robh e math sin?" (literally, "and was that good?")