"They serve fish and potatoes."
Translation:Riarann siad iasc agus prátaí.
Riar is a transitive verb where the object is the thing being "served up".
Freastail is a transitive verb where the object is the thing or person that service is being given to - freastalaíonn siad ar an bpobal - "they serve the public".
In English you need context to tell whether "They served Bacon" is a statement about breakfast, or about the people who work for the actor, Kevin Bacon. In irish, you use two different verbs.