James, I once knew very well the places you wish to represent in miniature. So I'm rooting for you.
You probably already considered the hard reality that, as much as the real places fascinate us, we cannot reproduce them in miniature. We sometimes, though, can represent those places with the most iconic features.
Thinking about the area you're describing, to me, the most memorable features are the Milltown bridge and the valley below. The distance between, say, Milltown and Dundrum is not necessarily a memorable feature.
Iain Rice wrote many inspiring, insightful books about representing the real world in miniature. Before him, an American named John Armstrong wrote about model railway design. You might want to spend time immersing yourself in them, if you haven't already. I'm doing the same.
You probably are already thinking this, but "selective compression" of a real place into a representative miniature design is what allows us to make something recognizable.
You'll also want to think about what you really want your end goal to "feel like." Do you want to run model trains from one end to the other, recreating Harcourt Street Station to Foxrock (or Dundrum)? Or might you want to represent the feeling of the railway line, perhaps by having an around-the-room set of relatively narrow modular layout sections (perhaps 12 inches to 18 inches deep), where the train leaves Harcourt Street Station, crosses Milltown bridge and the river valley, then pulls into Dundrum, then perhaps circles back past your terminus?
I look forward to your creative exploration.