thú ...great, yet another variation to add to the googolplex of others to attempt and fail to memorise how and when to use or not. I'm beginning to think I'm completely wasting my time here. There's just too much variation to hold in the head. I remember feeling like this before with Gaeilge but perhaps it's just time to finally concede that I'll never understand this mind-blowingly intricate and complex language. Thankfully I live in Ireland so I'll never need it.
Sorry, don't really mean that, I'm just feeling bitter battling with it tonight.
No, I'm pretty sure it's because you use the disjunctive forms of the pronouns with cén (that might be related to the lack of a verb), and thú is the disjunctive form of tú. https://en.wikipedia.org/wiki/Irish_grammar#Personal_pronouns