Is is the present tense of the copula (explained presently).
Maith is a feminine noun that means good.
Sin is a demonstrative adjective that means that.
Thus, is maith sin: that's good.
Slight variations include the form is maith é sin, which also means that's good but is just a little more gramatically specific, and the form is maith an rud é sin (that's a good thing).
Is maith sin is likely to be the source of the English word smashing (in the sense of 'great!', 'fantastic!').
The copula, is (present negative form: ní; present interrogative form: an), is normally used in Irish to express some essential or permanent characteristic of a noun or pronoun (or the lack thereof) by linking it to another noun or pronoun that crystalises that characteristic.
Consider the following examples. Note that the 'crystalising' noun or pronoun comes first.
*Pádhraic, pronounced /pɑ:ric′/, is the traditional Connacht form of Pádraig (traditional in Munster). It continues to have currency.
A slightly different positive-only form, which is valid within the official standard, is characteristic of Munster Irish:
Complementary to the copula, superficial, transitory characteristics are generally expressed using the verb tá.
Up to this point, we've looked at some simple uses of is and tá. If you're interested in some more complex and subtle forms, grammatically valid and with currency in naturalistic speech, read on. (Feel free to stop here if you're not!)
Is and tá can actually be used together. The following are all quite correct:
Note that there are subtleties involved with this form, with the focus shifting slightly from the object to the characteristic.
The copula is generally used to link a noun or pronoun to another noun or pronoun. However, there are some valid cases where it is used to link an adjective to a noun or pronoun:
Only certain adjective/(pro)noun copula are grammatically and pragmatically valid. Avoid the temptation to compose your own ad hoc.
The above is just a broad overview, and is not exhaustive. The interested reader is encouraged to find out more about these various forms, their subtleties and their correct usage.