The road from Larne Harbour to Portaferry

Distance: 49.77 miles, 80.1 km, about 1 hours 37 mins

From Larne Harbour take the A8 West heading to Belfast for approx 13 miles. At Glengormley bear left onto the M2 heading for Belfast, approx 6 miles. Follow the signs on the M2 for Newtownards, approx 13 Miles. 
At Newtownards follow the signs for Greyabbey on the A20. 
From Greyabbey follow the signs for Kircubbin. After Kirkubbin head straight on to Portaferry. The Ferry itself is signposted. Barholm house is at Portaferry seafront, directly opposite the ferry slipway.
Approx. 50 Miles. 

From Belfast take the A20 heading East toward Newtownards Approx. 13 Miles. Then same as above.

Barholm is opposite the ferry terminal, 
just where the green A2 box is on the map.

Don't go this way. You will miss the fabulous scenic road down along the side of Strangford Lough.

And then you will have to get the ferry across the Narrows. 

There will be plenty of opportunity for that later.