Re: Trip to Ireland approaching
I just got back from a trip, I had my G4, and my Nikon P7700 (an "advanced" P&S, for lack of a better description).
I took a bunch of pictures on the G4. But I used the Nikon for the "important" stuff, it was what I preferred to use.
One obvious difference is the lens. The G4's wide-angle lens (in 16:9 mode) is great for landscapes, it took the attached picture. But frankly, it leaves a lot to be desired if your subject is smaller/further away. You'll end up with a picture of a small subject against a large background
The SLR will likely have a more-useful flash, for when you're in dimmer lighting. It will also offer more useful modes (scenes modes, aperture-priority for depth-of-field control, etc). And the image quality should be better (better lens, larger image sensor).
The G4 was great to have in my pocket. I used it a bunch for quick-photo opportunities, if just walking around with my "real" camera still in its bag. The large aperture of the lens, combined with OIS, let it do surprisingly well indoors, if the subject isn't moving. Also, its HDR mode gives better results than the one on my Nikon. The GPS tagging of the pictures can also be nice, if trying to remember later where a picture was taken.
But I would have missed a lot of nice pictures if I only had the G4. Anything far away would have been just a spec in the picture.