I'd suggest that depends on your intended exposure to the currency exchange issue - will it be very occasional, or every month?
I have a euro mortgage (being a resident of the Irish Republic) on a property in Northern Ireland, obviously in the Sterling zone, with the Ulster Bank.
Having a good relationship with my bank manager definitely helped matters, but the process required some months of insistence on my part as her various superiors repeatedly told us that such a mortgage was impossible, and that I instead should take out a sterling mortgage with one of their Northern Branches.
I subsequently discovered that they were approving only a handful of such mortgages each year at that time (2004.)
This arrangement works for me primarily because I am employed and earn in the euro zone, hence I don't have the hassle or cost of exchange when paying the mortgage. Also, I am in the position to allow income from the house to accrue in a sterling account for repairs, etc, and in order to save for eventual renovation and extension.
However, if it were simply a case of seeking to make a small percentage on the interest rate differential, while at the same time moving mortgage money or rental income money back and forth across the border each month, then you're on for a hiding. The latter will greatly overrun the former, and you'd have been better off with a sterling mortgage. Any of the banks with operations in the UK and Ireland are able to provide sterling mortgages to Irish residents (and in this instance and my experience are likely to seek to foist one on you anyway.)