As long as you drink plenty of water you will likely not have a hangover the next day, or if you do it will be a very small one.
At the ICB tastings I have taken to drinking the water we use to wash our glasses with instead of chucking it in the Stein.
This hydrates me making me feel fine the next day and also the stein does not fill up so fast.
I have heard of people having immediate headaches with Guinness. My brother said he gets them. Maybe an Iron thing? I know Guinness is not overflowing in Iron but if you have too much Iron in the system already, which is possible with my brother then that might account for it since Guinness does have some iron.
Not being a medical expert I can not say for certain though.