Little calculation:
A forum will give about 0.02 to 0.16 more money income per month, depending if you can trade with other countries or not.
An irrigation will give 50% more tax income in one province, so if the province has about 6 slavepop (average number for a province with 18 pop) it will increase the tax income by 0.125 per month
Stability will increase tax income by 10% for each of your provinces, so if you empire has about 70 slavepop money income will be increased by 0.3 each month.
So the solution is:
First build irrigations in the provinces with more than 7 slavepop, then build forums in provinces where you can trade the tradegood to other nations. Then build irrigation anywhere else, where it's possible.
Stability should be increased before you get a huge amount of provinces in a short time (especially in a conquering war) Because it will get much more expansive afterwards. Stability should never be increased before an event that will lower it, a dow without cb for example.