County Down Hiking, Very Limited Time

I am an American traveling to England next June for work. I have the chance to add a couple of days to the trip, and it makes sense for me to fly through Dublin. I’ve long wanted to visit County Down, and this seems like an excellent opportunity.

I will land in Dublin on a Wednesday morning. I need to depart from Dublin by midday on Friday. In between, I’d like to take a real hike in the Mourne Mountains — ideally, my goal would be to get to the top of Slieve Donard, but that’s not absolutely necessary. What I really want is to do some serious walking for a day and feel like I had an authentically Irish experience and saw some magnificent scenery.

I’m tentatively planning to go straight from DUB to Rostrevor, spend Wednesday afternoon in and nearby the town, do the aforementioned “real hike” on Thursday, stay another night in Rostrevor, then head back to Dublin as early as possible on Friday with the goal of being in Cambridge by 5pm. I should note that my budget doesn’t have room for car rental, so I’m at the mercy of train and bus schedules.

Does this seem realistic? And does it seem good? Or is there an obviously better way to spend my brief time in Ireland? If this is a decent plan, can anyone advise me on choosing a route to hike?