You can build a city on a hill to transmit irrigation across the hill, if you can irrigate up to the hill and have irrigable tiles (flat grass, plain or desert) on both sides of the hill.
If I recall, this works in Conquests and PTW but not the original CivIII. I've done it recently on Conquests.
Or research to Electricity in the Industrial Age and then you can irrigate without fresh water.
Or restart the game.