Employed in England and living in Ireland, USC exempt?

I have recently received a note from revenue.ie that i am liable for USC. I work in england on a 4 week on 4 week off basis, and i am living in ireland. I pay tax and NI at source in the UK. Am I still liable for USC?
Does anyone have any contacts for a tax specalist that can resolve this, as 4 accountants close to where I live will not touch this as they call it a "grey" area.
Thanks.