Costs for installing a downstairs loo

[Donegal Holiday]

We are buying a house in Donegal and want to install a downstairs loo. We may need to build a tiny extension to house it. I know this is a v difficulty question to answer, but does anyone have any idea how much this would cost? I live in England so have no idea re labour costs in Ireland. I'd be really grateful just for some ball park figures/anyone else's experiences of this kind of job.
Cheers.