You're talking about marches, not swamps. Swamps was in Civ1 and 2 too iirc.Been a while since I've played, but I seem to recall that in C3C you can irrigate a marsh tile directly, rather than clear it and then irrigate it. New install, new machine, and I don't have my old saves so I can't check it.... Am I just dreaming?
Irrigating a marsh/swamp would be redundant like chopping down a plains! Or flooding an ocean.
No, you got to make marsh into grass first. Has been since Civ1.
Being a close-to-marsh (or jungle) civ can be a pain, I was the Iroquois once and had to clear what felt like a bazillion tiles to expand properly.