I second what Kreutzer says above. I just got back from a month-long trip to India and Nepal, and brought a 5D3, 17-40L, 70-200 f/4L IS, 50mm f/1.4 and 2X extender with me. I left the 24-70L and 70-200 f/2.8L II at home but didn't miss them for a minute. When I visited Nepal I hiked up to Everest Base Camp, where keeping the weight of my daypack minimized was crucial, so I left the 50mm and 2X extender behind in Kathmandu and only traveled with the 5D3, 17-40 and 70-200 on the trek. Got some amazing photos and was able to travel relatively light at the same time. Using the 70-200 f/2.8L would obviously be heavier but like someone mentioned earlier, Ireland can be dark sometimes due to the weather. It was sunny most of the days where I was at so I didn't need the larger max aperture...
I visited southwest Ireland back in 2001, although at the time I was just starting to get into photography... you are in for a treat considering the location and the gear you're going to bring. Have fun!