You are correct that technically it is +50%, which is only noticeable for special tiles that have more than 2 food (Wheat, Oasis, MuskOx) and can be irrigated (Pheasant and Spice have 3, and Fruit has 4, but irrigation changes them to Plains and Grass). In testing this I discovered you can double-irrigate Tundra and Hills, but you need to start with 4 food (3+1 for first irrigation) in order to get more than +1 effect.
I also discovered Cheat mode will let you put irrigation on a Mountain - don't know if that's useful to anyone...