That's not quite right. 'Tá' only means 'yes' when answering a question in the present tense that uses the verb 'bí', e.g., 'An bhfuil tú anseo? Tá.' ('Are you there? (I) am' - 'bhfuil' is from the dependent form of the verb 'bí').

OIr 'tó' and 'tá' aren't (directly at least) related. From what I can dig up from eDIL, 'tó' and 'náthó' are borrowings of the Welsh 'do' and 'nado' that later fell out of use again.