How do you pronounce and write yes and no in Irish?

Technically, there are no Words in Irish for 'Yes' and 'No' however, most speakers would use 'Tá' pronounced ('taw' rhyming with 'paw') for yes, and Níl (pronounced like 'knee')

People do use these, but because grammatically speaking they are used as positive and negative dimunitives, they don't really Word alone.

a grammatically appropriate usage would be

tá mac agam ('I have a son)

Níl mac agam (I have no son)