"I want to work in a garage."
Translation:Dw i eisiau gweithio mewn garej.
There is no difference in meaning between the patterns.
Yes, eisiau is a noun and the pattern dw i eisiau... is probably a simplification of the more formal but uncommon rydw i ag eisiau... where the ag has been dropped.
moyn is a contraction of the verb-noun ymofyn (which has some wider meanings than just 'wanting') and so yn/'n (or wedi in some tenses) is needed as a link to dw i, etc.