To make that more clear:
If a tile would produce 3F or more in any government but despotism (and maybe anarchy, I don't know) then it produces one less instead.
So irrigating would normally give you a 3F 1S 2G tile, but in despotism this is reduced back to 2 1 2.
However, a normal floodplain is a 3.0.2 (because it's always on a river) except for in Despotism, where it is 2.0.2. So if you irrigate it it becomes a 3.0.2 again. A normal plains tile is a 1.1.0 tile, but if you irrigate it it doesn't matter what goverment you're in, it will always be 2.1.0, as it doesn't hit 3F.
All two-food tiles should be mined in despotism, except those who would actually produce 3 food out of despotism, you can irrigate those to get a despotism 3 F tile. To check how much food a tile would get outside of despotism, simply go to the Civilopedia.
I have a question for other members though: does this apply to shields aswell? And to commerce?