There is often a lot of confusion with the verb to be, which is expressed using the word tha and the word is.
Tha mi fliuch I am wet
Is mise Joseph I am Joseph
There are some people who believe that the IS version is archaic and is starting to disappear and indeed it sometimes does from sentences. But it is still used sometimes invisibly.
The best way to know when to use them s simply to think about emphasis – if there is a physical characteristic like a name, its unlikely to change so you can use is mise, but for occupations or transient things we use Tha…… although for vocations or people who feel they have a gift or a special calling, we can also use Is.
There is a very helpful chart here