First off: Nice! Ireland is on my bucketlist as well
Second, what is your main goal during the holiday? Will it be city hopping or driving around countryside?

Personally I think the 18-135 sounds like a good do it all lens for general shooting and city-hopping. I would definitely bring the 100-400 if you will go into the country-side more and if you don't mind the extra cargo you carry along.

In my most recent trip to norway I took the 16-35, 24-70, 50 1.4, and 135 2.0. And our fellow travellers too their 24-70 and 70-200.
I took about 1200 shots in total. I ended up with 137 keepers out of which:

103 with the 24-70mm
4 with the 50mm
22 with the 16-35mm
5 with the 70-200mm (friends)
3 with the 135mm

So basically I could probably have been taking photos with only the 24-70. There were only a few times where I "needed" the other focal lengths.
This was on a mostly hiking/nature-trip so it might not be entirely relevant to you, but it might give you an idea.

For city-hopping I would stick to the 18-135 and maybe add a wider lens.
For more nature/country-side you might want to carry the 100-400 along.