Hi Micca - transport info here with a link to Ulsterbus' website.

Belfast has two main bus stations: Europa (Glengall Street) and Laganside (Donegall Quay).
You get the airport bus from just outside Arrivals at the airport. It takes maybe 30 - 45 minutes to get to Belfast. It will probably go to Europa but you need to catch the bus to Portaferry from Laganside. There may be a linking bus between the two bus stations, but it's not that far - it can be walked. When getting the Portaferry bus at Laganside, try to get the more direct one - the one that goes via Kircubbin, rather than the long, windy, tedious one that goes via Portavogie or Ballywalter. Also, ask if you need to change at Newtownards (or in "Ards", as the town is also known). I think there should be direct buses via Kircubbin, but Newtownards is the biggest town along the way, and there's a bus station there that the bus might stop in for a while.

In Portaferry, the bus will drive into the town and will come to The Square - identifiable from the shape and the big Market House and a bus shelter. From there, walk behind the Market House and take the road that goes off to the right, downhill towards the sea and the castle (it will be signposted either "Castle Street" or "The Shambles"). Follow it down to the seafront and turn right. Barholm is on the right just past the lifeboat boathouse.

Note - for anyone driving, the Portaferry is one big one-way system. I think I posted details about this earlier.