I believe the house has to be super-efficient and insulated before these heat pumps can make a difference. So for every 1kW energy you put into them, you get 2-4 back. But if your house has a few drafts or the attic isn't insulated properly, it all goes back out in no time. I believe your house has to be A2 or A3 rated for them to make a difference. And only houses built in the last few years would be that good.
The €700 ESB guy was probably mis-sold a heat pump by some chancer who promised the earth.
I think for most people in the average house, upgrading the attic insulation & the windows and eliminating drafts is as good as it gets. Oh, and maybe a new boiler if you can afford it.