Táim‑se is the emphasised present tense, first person singular of the verb be, meaning be.
It is a coalescence of the verb itself, the personal pronoun mé (me) and the emphatic suffix ‑se.
Thus, táim‑se: I am.
Note that in Irish, emphatic suffixes are not themselves uttered with stressed emphasis. The stress is placed on the word to which they are appended.