"Fog" is a type of weather?!?!?

There are different kinds of fog. A fog is just a cloud on the ground. The fog that you commonly see in the midwest is cause by high humidity collecting at ground level and as the day heats up evaporates. All coast areas the fog is heavy, cold air over the sea that is pulled in all day as the air heats up over land, raises, and is replaced by the cool air over the water. You can have fog all day, all night, and maybe all week.

In California we have "ground fog" sometimes called "tulle fog." It can be thick enough that driving is hazardous. Visibility can be nearly zero. If you are driving in Ireland or anywhere else, be aware of reduced visibility and adjust your driving speed accordingly. If you can't see even one car-length ahead, don't drive. We typically have one or two multi-car pileups in California every winter when there is thick ground fog. People die. I've heard of this kind of fog in Germany. I would expect fog in Ireland to be less hazardous for visibility, but that is just a guess.

No worries about driving. We are NOT driving over there! :)

I've seen no sign of any fog over here. Yes most of the time it will clear to become a nice sunny day, very rarely will it persist throughout the day. I wouldn't worry too much about it.

In the cooler months, fog may linger throughout the day, with the most dense layering occuring in the morning and evening -well, at least in Belgium, France and the Netherlands, I assume a similar pattern holds in Ireland. But I suspect that in September, it shouldn't last all day.