I expect I.F. will follow P&O's approach of passing on the extra cost to the passenger. I think I.F. are lucky enough on the French route, as if the going gets too tough with the low sulphur fuel, they can always charter or sell Oscar.
At 27 she's no spring chicken, though she has been very well looked after, especially by CL. I'd say Oscar has a good few years ahead of her, but if these will be in Irish waters is debatable.