It sounds like a lot of climbing to me.

It's too bad there is no shorter option. One thing you could do is make sure in advance that there is a broom wagon, and then start the ride, hope for the best, and be aware that you can use the broom wagon option if you feel you can't finish. However this only works if you're okay with the possibility that you won't do the full ride -- some people feel very disappointed and frustrated if aren't able to finish.

What specifically is the ride you're thinking of doing? I have a cycling friend who lives in Dublin, and I can ask her if she's familiar with it.