We exported a dozen ewes to NI, and it was a huge faff and definitely not worth the bother. We delivered them too, but could not have got them to Eire in the 8 hours our license covers. If you are going to go ahead with the sale, make sure the buyer or their haulier collects.
There will be a border inspection when you arrive, or in NI there is, and that has to be factored into the timing too.
We found it an expensive luxury, and unless your flock is already scrapie genotyped, MV whatevered and so on, then you will spend an inordinate amount of time on this.