As an experienced semi-pro (I do sell prints, but the majority of my income is not derived from it), I think I am best placed to answer this question so far.
First, I don't know how experienced you are with backpacking, but in general, the lighter your pack weight, the more enjoyable the trip, all else being equal. Some clearly inexperienced people have posted that you should replace the 2nd camera instead with spare clothing/socks/underwear etc. No. No matter what, you should have spare clothing.
Second, your day is not going to be filled with scenes that will drop jaws. You will have time to switch lenses. What I do is carry a small point and shoot for those scenes I want pictures of, but clearly won't be good enough for printing. Most pictures will reside on your hard drive and nothing more. Some will be shown on the internet. Even less will actually be printed. If you have even a bit of experience, you'll know when you take the photo if it has potential or not.
Third, in about 30 years of doing photography, I've never had a camera fail to work for me. It is so incredibly rare, that if this is your worry, you're likely never to experience it in the field and don't need a backup.
Fourth, KISS. Never heard of it, I'm guessing? It stands for Keep It Simple Stupid. Two bodies and multiple lenses is not only heavy, but complicated. When you have so many options, it can get confusing for you beginners about which lens + body to use. If you miss a shot, then you it bugs you that you may have gotten it with another combination. Or the added weight is tiring, and you don't take the shot because you are tired, but then you might be filled with regret about the lost opportunity. One camera, one or two lenses. That is a successful formula. The landscape photography mentors I have all use the same approach, and they do this for a living, where their livelihood is based on the work they produce. Since most people don't follow advice until they learn the hard way anyway, I recommend you try taking two cameras (if you have them) and multiple lenses. You'll find that you have one workhorse lens, another that you use, and the rest don't even get touched. Figure out which lenses those are (some like ultra wide angle, some like telephoto, etc), and then bring that one or two with you.
I myself use the world-class Canon EF 16-35mm f/4 IS USM L myself. I either bring only that lens, or with the Samyang 14mm Ultra Wide-Angle f/2.8 IF ED UMC if I will be doing star photography, or the Canon EF 70-200mm f/4L IS USM if I think there will be good telephoto opportunities. Remember, a maximum of two lenses. This keeps weight down. This increases your enjoyment while you are on trail, which is an important mindset to maximizing your creativity.
Now, if you do other types of photography than just landscape, you are more than welcome to have more than one body. It can be very useful in event photography, for instance. But for backpacking, one camera. Of course, when I say camera, I refer to DSLR. In reality, what you should have is one DSLR + one P&S.
Good luck! Please post pictures once you get to Ireland! That's definitely a place we all have on our bucket list. Gorgeous place from what I've seen!