It is normally "cad chuige an bád san?", or at least that's what I think

and then it is an expansion of that with "ab ea". It doesn't look grammatically incorrect.

Hi everyone!

I was listening to yesterday's episode of an saol ó dheas wordle game and heard "cad chuige" in usage by Helen Ní Shé.
I know for other parts of the country you can use "cad chuige" + "indirect relative clause" as a drop in replacement for "why" e.g. cad chuige a raibh sé ann?
And in Munster I know the idea is to use it for the sense "what purpose?".

However, I'm not entirely sure of the correct grammar given Helen's usage.
While talking about a boat, she says: "cad chuige ab ea an bád san?" = "what was the purpose of the boat?/what was the boat used for?".
This seems to use a copular clause which contrasts against the relative clause given above.

So is it the case that cad chuige is considered a noun and then we use a copular clause? e.g.
- cad chuige ab ea an bád san? = what was the purpose of the boat, what would the purpose of the boat be?
- cad chuige is ea an bád san? = what is the purpose of the boat?

Bonus question - how do we express usage/purpose in the future tense? (as I believe the. copular does not support this)