About this phrase

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).

Smashing!

Is maith sin is likely to be the source of the English word smashing (in the sense of 'great!', 'fantastic!').

The copula (is)

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.

• Is fear é Tomás: Tomás is a man.
• Is bean í Caitlín: Caitlín is a woman.
• Ní fear í Síle: Síle is not a man.
• Ní bean é Pádhraic*: Pádhraic is not a woman.
• Ní dochtúirí iad cócairí: cooks are not doctors.
• An ealaíontóir í Bríd?: is Bríd an artist?.

*Pádhraic, pronounced /pɑ:ric′/, is the traditional Connacht form of Pádraig (traditional in Munster). It continues to have currency.

The copula in Munster Irish

A slightly different positive-only form, which is valid within the official standard, is characteristic of Munster Irish:

• Gadhar is ea Rex: Rex is a dog.
• Banaltra is ea Gobnait: Gobnait is a nurse.
• Siúnéar is ea Conchubhar: Conchubhar is a joiner / carpenter.
• Bogha leaca is ea é: it is a rainbow.
• Taibhreamh ab ea é: it was a dream.
• Feirmeoirí ab ea sinn: we were farmers.

The verb tá

Complementary to the copula, superficial, transitory characteristics are generally expressed using the verb tá.

• Tá Máirín óg: Máirín is young.
• Tá Siobhán ina codladh: Siobhán is asleep.
• Tá Seán faoi bhrón: Seán is sad.
• Tá an pláta te: The plate is hot.

More complex constructions

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 used in conjunction

Is and tá can actually be used together. The following are all quite correct:

• Is óg atá Máirín: it is young that Máirín is.
• Is brónach atá Seán: it is sad that Seán is.
• Nach liom‑sa atá an t‑ádh?: isn't it with me that the luck is? (essentially, 'aren't I the lucky one?').

Note that there are subtleties involved with this form, with the focus shifting slightly from the object to the characteristic.

The use of adjectives with the copula

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:

• Is óg í Máirín: Máirín is young.
• Is geal é mo chroí: my heart is bright (essentially, 'I'm very happy').
• Is mór an trua é: it's a great pity.
• Is bocht an scéal é: it's a great shame (literally, 'it's a poor story').

Only certain adjective/(pro)noun copula are grammatically and pragmatically valid. Avoid the temptation to compose your own ad hoc.

Further study

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.