Hi just a quick observation: Bibor you might want to test and add this if my theory proves to be correct.
A lot of people are complaining that French forces don't attackt their allies, I believe this might help change it:
By the time you have retreated your troops to the mainland and are forging claims against the Irish minor, the French are sieging all your provinces,
with its big stacks in the North provinces and small stacks in the south provinces. If this happens and you take your allies into the war, France usually won't attack them but rather patrol with its doomstack.
The way to fix this is: Take your 14stack that you use to annihilate small forces and head down to the south provinces, while yes you need speed to not go below -25%, it is not necessary for France to have sieged all 4 provinces. The trick is that France sieges down the North provinces, and you attack the south ones, annihilating one of the batallions that was sieging your province, so while all other 3 are captured, one is not. Your attack will lure the doomstack to the south, if you head straight back to the mainland, it will siege the 4th province, and the doomstack will stay in the south.
Now distance to an enemy, i.e. the proximity of the AI's army to the oponent seems to play a very VERY important role in AI decision making. Thus having the doomstack near the Spain border, when you declare war on the Irish minor and call in your allies, the doomstack will head straight into the Iberian Peninsula where it will most likely be annihilated.