PK,
If you are on the floor near the outside of the ellipse or inside the ellipse.. I would NOT USE A FLASH! Imagine having thousands of flashes going off in your eyes! It's not very kind to the boys. If you are using a film camera use a faster speed. Like 800 film. That way with the lighting and movement you will get a better picture. If you are using a digital camera change your settings to the sports mode, higher shutter speed, i.e. ISO to about 400. Fstop to somewhere around f3.9 I think. Also, if you have an auto stabilization mode on the camera use it due to everyone/all the movement around you it's hard to keep the camera still. You won't need a flash if you are able to manipulate the settings on your camera.
Another note: U2 Management's Policy on Cameras are they are allowed except for professional cameras with detachable lens. I would check with venue staff and make sure they are aware of this and to make sure they will allow cameras. Lot of times the two groups don't communicate policies to each other so venue staff has said no to cameras but then had to be corrected on the matter. For example, back in April at Staples Center in LA, on the first nite of two nites venue staff made everyone take their cameras back and were not allowing them due to them being unaware of U2's policy. The next nite they allowed cameras.