hi mazie,
this question normally gets jumped upon with the answer "how long is a piece of string ?" and although it's frustratingly unhelpful, it is a very important lesson to take on board very early on.
There are a multitude of factors which can have a bearing on your home build project.
The key to it all is to do plenty of research initially and get as much knowledge of and familiarity with the whole process as you can.
A 4 bed house could be 120 m2 or 300 m2 with widely different build costs.
The site could be problematic and require a huge amount of excavation and costly groundworks before any building even starts.
Any variations from a simple rectangular box design, has impact on costs.
The build route you choose, main contractor, sub contractors managed by you or a project manager ...
Where in the country you are located, build costs are significantly different in Dublin and Donegal.
And the list goes on.
Having said all that there are still some guidelines which will give you a ball park idea of what a build could cost.
On paper, roughly draw a simple plan of the rooms you would like in your house and give them sizes (length and width). Work out the area of all rooms combined, add 15% and see what you end up with. I guarantee it will be bigger then you thought.
Take a very general figure of €100/sq ft (as I said this could vary a lot).
This will give you your basic build cost, a house finished to a certain point, usually called a builder's finish (although this can also vary in it's meaning).
Now work out what you want to spend on your kitchen, bathrooms, flooring, tiles, curtains, blinds, furniture, landscaping
After this you will have an idea of costs to start with, but as I said be aware of all the other factors involved.
Hope this helps.
PS: I'd love to meet the person who said they could build a 4 bed house for 80k.