Hi Gavin,
the current maximum income tax rate in Sweden at present is0- 56%, which I believe includes a social insurance payment element. I am still adamant that you will have to pay Swedish income tax but not the social insurance element. I would advise you to get advice from a Swedish international tax advisor/ accountant.
The swedish social welfare system is financed primarly from taxes, but you should not have to pay the social insurance element, when you are already paying it in Ireland as you are entitled to do. I do not think that the Swedish employer will let you remain working if you do not register your details, before you do this you really need to get expert advise on this one.
Wish I was more help