A little bribery will get you the irrigated square back (assuming they didn't pillage while they were there). But under democracy you need to be ready for a couple extra sad faces from the bribed unit.I find it extremely annoying when a supposably peaceful nation plants a unit in an irrigated tile near my city
I've always liked monarchy. I tend to jump to it as soon as it's discovered and then stick with it.
If you have no enemies nearby, Democracy is by far the best.
Good luck!
Just curious, why is democracy 'by far' better than republic? AFAIK it only eliminates corruption. Corruption was never a big issue in my games.