They have to re-route you free of charge under comparable transport conditions. If the rescue flight isn't under comparable transport conditions, then they can charge you extra for the rescue flight. If the rescue flight isn't under comparable transport conditions, then you can insist that FR pays your hotel and restaurant bills in the UK until such time that it becomes possible to travel to Ireland under comparable transport conditions. I'd imagine that it is cheaper for FR to pay for your rescue flight than to put you in a hotel for a prolonged period of time.