Superman, I don't see Monaghan on that list.
The reason Monaghan is (potentially) much cheaper is that its a border county and NI builders are willing to commute to building sites from their homebase up north. And in the Wee North nobody pays the extortionate building rates we are screwed for in Dublin, or points further south.
Half price is TOTALLY realistic.
Now, you tell me where I can get an Architect to design (and get planning, and supervise/sign-off) my one-off 1800 sq ft detached house in Dublin for a fixed price of 20K and I'll hire him on the spot.