"You prefer stew."
Translation:Is fearr leat stobhach.
stobhaigh is the genitive of stobhach. They are not interchangeable.
Is maith liom stobhach - "I like stew"
babhla stobhaigh - "a bowl of stew"
The genitive is also used to form an adjective of a noun, so "stewing beef" (the kind of beef used to make stew, not the process) is mairteoil stobhaigh