Counterspell reads, “Secret: When your opponent casts a spell, Counter it.”
Marshspawn reads, “Battlecry: If you cast a spell last turn, Discover a spell.”
So it seems to me that Marshspawn’s battlecry should trigger even if you cast a spell last turn that was Countered. However, this is not how it works. If you only cast one spell on the previous turn and it was countered, Marshspawn’s battlecry doesn’t trigger. I had this happen on Ladder and just confirmed it in an Innkeeper game. Is this interaction working as intended or a bug?
Edit: Just tested to see if you cast 2 spells on the previous turn, but 1 was countered if Marshspawn’s battlecry activates and it does.