"I love fashion and history."
Translation:Is aoibhinn liom faisean agus stair.
The definition of an in the FGB includes "5. (With abstract nouns, in general reference)", and gives the examples of an ceol for "music" and an t-ocras for "hunger", etc. Languages and Educational subjects also take a definite article when used in this way (when they are used "in a general reference"), which is where you got your an stair from, but if you are treating stair as a general topic/subject in this exercise, then "fashion" would aso be a general topic/subject, and therefore be subject to the same rule. So the "class" doesn't have a grammatical name, as such, it's just a group of simiilar things that are subject to a specific rule.
Because this is an aspect where Irish and English differ, and the "rule" isn't always easy to apply (how do you tell whether any specific example is "a general reference" or not?) you will find examples where an is used, and other examples where it isn't, but within any give example, it should be used consistently.
I honestly don't know whether stair or an stair is better in this case.