Hi,
Can the DVLA tell you if the registration is an age-related plate? (it sounds like it is)..
The problem with us (the Irish) is that we've been leaving the country regularly for centuries and tend to turn up in far-flung places so it'd be hard to pinpoint the car's origin that way. Is it possible that your car was purchased by a member of the forces who exported it tax free while on duty abroad?
I had a look at
http://www.volvop1800club.se/eng/ where you posted details of your car. Chassis number 27884 (also light blue) is also a UK car but unfortunately there's no UK car in the number sequence above yours before 27904 which is a LHD car in Costa Rica - I was hoping to find a UK car on both sides of your chassis number which might show that they were in the same batch. 277884 is a G reg
The problem with the Irish vehicle registration system is that it was only centralised in 1993 - I'm not sure that you could find out much about your car's possible existance in Ireland without knowing which county it was registered in, so catch 22 all in all I think...
Your best lead seems to be the deceased owner. Can you trace any relatives who might remember something?