Where to get an electric boiler for just 3 taps and a shower?

How is the space heating being done, is that electric radiators too?

There are options for electric water heating, including some with storage, but the running cost is high and unless you have considerable water storage (a large cylinder) you will have to compromise on things like flow rates.

Gas is an often a much better option. If you look at Rinnai water heaters. This could be run from a gas cylinder and even mounted outside.

In that case, if you look at Atlantic water heaters. A 30 litre unit should comfortably do the three taps.

Then a standalone electric shower like a Triton T80

Yes, a 30 litre isn’t going to run a shower. But you have options;

If you had space to install a larger one of those Atlantic units, like a 200Litre, you could feed your taps and also run a high performance shower for up to 20 minutes, before the water heater would need to reheat, which would take about 4 hours.

An electric shower doesn’t need storage, it heats on demand. But the water flow will be much less (approx 1/3 of the flow) than can be achieved with the above.

I am assuming this cabin will have its own dedicated ESB supply?

but he'd have to make **** of the walls which is the last thing we want before we're even able to move into the place we've spent our life savings on!

So the cabin is built? The time for deciding all the utilities etc was at the design stage, all the necessary plumbing and wiring could have been done during construction and you wouldn't need to rip up anything.

Anyway, no point in crying over spilt milk now, your options are limited now it's built and you don't want to rip it to bits again.

What stage is it at now? Is all plumbing and wiring done already?

I have a 10 litre ariston undersink water heater feeding the kitchen and two bathroom taps also working off it. Its left on all the time. I have a directly heated shower.