Thanks all!
I'll try Caledonian and Allianz to hopefully get three quotes to compare.
I disclosed to the insurers that it is a protected structure when I was looking for a quote. Some companies have it as a question on their forms, but even if they don't specifically ask it I'd imagine that it would count as a 'material fact' that could invalidate the policy. Many of the online forms ask that you call for a quote if a house is over 100 years old and then check with their underwriters before quoting.
Yes, it's a normal permanently occupied house. 'Protected Structure' is just the term here for what people refer to as 'listed' buildings. A surprising amount of the housing stock in central Dublin is covered by this, see the full list of protected structures here (~9,000 entries in DCC):
[broken link removed]
Thanks,
Circle