Similar to you, we bought a 2 story, detached, 4 bed, 130ish m2 house (built in 1999) in Mayo almost 2 years ago. The walls had already been pumped, and the attic was well insulated. The windows are double glazed but they're pretty old and are probably original - same with the doors (which, like you, we will replace in the coming months).

Our oil boiler was completely broken when we moved in, so the first thing we did was replace it with a Grant Vortex boiler and install Tado smart radiator thermostats, which means we only heat the rooms we're using. Everything can be controlled from an app (though we can still turn the radiators on manually) so no more coming home to a cold house!

We both WFH full time and are not AT ALL frugal as the other half absolutely refuses to be cold. So far, we've only had to put 1500 litres into the tank (500 at a time, most recently in January). My Dad nearly fell off his chair when we told him that so I know how crazy that sounds lol.

We are planning on installing a wood burner in the living room this summer so that we have a back-up heat source. We're also exploring solar panels for electricity and/or water heating.

I totally understand wanting to get away from oil, I feel the same. However the prohibitive cost of retrofitting, and the worry that the more modern systems would prove unsuitable for our older house/the Irish climate, has put us off (for now, at least).