The pattern with eisiau is a highly irregular one in Welsh. It perhaps comes from a more formal pattern which has become shortened through long and frequent use:
- Dw i ag eisiau (rhywbeth) - literally, ' I am with a want (of something)', where the ag Is dropped in the spoken language.
You will very occasionally see the version with ag in writing, although there is a different pattern again that is more common in formal Welsh.
As mizinamo says, then, eisiau is not a verb-noun such as hoffi (liking, to like) but a noun.
The "Yes/No" answer to "Do you want?" is also irregular, isn't it?
At any rate, I've seen Wyt ti eisiau ...? answered with Nac oes. rather than Nac ydw.
Are both variants (Oes./Nac oes. as well as Ydw./Nac ydw.) common and correct?
The usual simple form is to answer with the same basic form in which the question was asked:
- Wyt ti'n mynd i'r dre heddiw? Ydw/Nac ydw.
- Wyt ti eisiau sglodion? Ydw/Nac ydw, diolch.
With eisiau, though, some people answer the informal question as though it been asked using a more formal pattern:
- Oes eisiau sglodion arnat ti? Oes/Nac oes. - a more formal pattern of the question, and the answer as expected.
- Wyt ti eisiau sglodion? Oes/Nac oes - answering the informal pattern as though it had been asked using a more formal pattern.
This last pattern is common in north-west Wales at least. If you go to any of the major centres of work and education you will hear both, of course, since people move around a lot nowadays.
The place I heard Wyt ti eisiau? Nac oes. on was Rownd a Rownd, which is set in NW Wales. (Filmed in Menai Bridge.)