Is the stamp duty payable on sites the same as for houses. ie. first €125K exempt and then 7% on next €875K.
And is the Duty payable on the total cost of the site and the build?
"Is the stamp duty payable on sites the same as for houses. ie. first €125K exempt and then 7% on next €875K."
No - its a different scale entirely. You should read Revenue leaflet SD10A for a full breakdown of stamp duty.
"And is the Duty payable on the total cost of the site and the build? "
As above. There is no stamp duty on a build if you already own the site. There may be stamp duty if there is a purchase of a site and a contract to build.
You should read SD10A and I also suggest that you talk to a solicitor to get a better idea of what is involved.