The reason why its eisc instead of iasc is because the "fish sandwich" part falls into what we call the "tuiseal ginideach"(tg) or the Genitive Case. The literal translation of "ceapaire eisc" is the "sandwich OF fish". That little word of causes the iasc to change to its tg form which is eisc.
I'm not sure if this will be addressed later in the course, but I have a question about how to use this sentence. I realize I might be making this more complicated than it needs to be, but I would like to become truly fluent, rather than passively fluent in this language (and I realize duolingo is just a starting point for this goal). You eat a fish sandwich. Could I say this as a command, like if a friend of mine was working really hard (learning Irish, say) and looked like they needed food, could I say ""Itheann tú ceapaire éisc." like a command? Or could I perhaps be a waitress and see a man I wait on? Perhaps he eats the same thing for lunch everyday, and I say ""Itheann tú ceapaire éisc!" I guess I'm looking for a more nuanced explanation of this construct (if there is one). Also, now I kind of want a ceapaire éisc of my own now. Thanks in advance!