David M is on this trip as the foreign policy advisor. He is a serious heavyweight in fp. They would send him to make sure the two stoners do not screw up and to watch them and report back. He does not travel with Harry and while Ireland is very important and a sensitive place my guess is the presence of MM is what got him this job. Bet he is not thrilled to be a babysitter. The foreign office must have been worried about screw ups to send him for damage control.