That's why you shouldn't rely on forvo for pronunciation - both of those recordings have a terminal K sound, but because of the different ways that different users recorded and edited their own recordings, that can get lost. You are better off relying on the recrdings at teanglann.ie
Irish spelling is a far more reliable guide to pronunciation than English spelling is, but there are aspects of it that just don't come naturally to English speakers, but as you become more familiar with the sound of Irish, these differences become more obvious, and you can tell when something doesn't sound right. And of course there are regional variations that can be quite pronounced, and confusing if you're not expecting them, because there is no single "standard" pronunciation
Having said that, if you are starting from scratch, Karen Reshkin's video can be helpful.
I was curious about the etymology of "iasc", wondering whether it was cognate with the Greek word "ichthys" (found in "ichthyosaur"). Well, whether it is or not, the etymology is very interesting: https://en.wiktionary.org/wiki/iasc. Turns out it's cognate with the English "piscine". However, most fascinating, it's remarkably close to the Proto-Indo-European "pisk" (meaning fish).
Yes, the Irish for "fish" is iasc. The first letter is capitalized in Duolingo exercises, so the capital "i" could look like a lowercase "L" in some fonts. Duolingo changed their font a while back so the "L"s are now both distinct from the "i"s (the lowercase "l" has a little hook at the bottom).