In despotism any tile producing 3 or more of anything per turn is reduced by 1. So anything that produces 2 is not worth improving to produce 3 because it will drop back to 2. But if it produces 1 or 3 or more then improving it will increase production to 2 or 4 or more.
Grass on its own produces 2. Irrigate it and it produces 3. Reduce these 3 by 1 for despotism and you get back to 2. Irrigating plain grass is a waste of worker time.
Grass with deer produces 4. Irrigate it and it produces 5. Reduce these numbers by 1 for despotism and you get 3 and 4. So irrigation works.
Grass on its own produces 2. Irrigate it and it produces 3. Reduce these 3 by 1 for despotism and you get back to 2. Irrigating plain grass is a waste of worker time.
Grass with deer produces 4. Irrigate it and it produces 5. Reduce these numbers by 1 for despotism and you get 3 and 4. So irrigation works.