Eric, are you not arriving at Belfast international till 2130? If so, you wouldn't make it to Portaferry that night, unless you cadge a lift or get a taxi. If there are two or three of you, hire a car - some firms have pretty cheap rates (search the web).

If you're landing around 1730 as you said earlier, you would be right to think you'd be at Portaferry around 2130: airport bus to Belfast Europa bus station (or get off as Laganside bus station if that comes first, as I think it does); then 1940 bus to Newtownards (1950 from Laganside), arrives N'ards 2020, then 2025 bus to Portaferry.

At other times of day it can be a bit easier than this, but whatever you do, don't even think about the ard mhacha route! It's not even really St Pat's grave at Downpatrick (though Thomas Russell's seems to be the genuine article).