Checked out Ulterbus, the service to Downpatrick from Belfast, Monday to Friday, Buses leaving from Europa Hotel area, on the hour from 10am until 2pm, and from 4.05pn and 5.30pm, the journey is approx, 45 minutes.
On arrival at Downpatrick you will have another 40 mins journey to Strangford, the problem with the bus service to Strangford is, there is only one at 4.05pm, monday to Friday.
The Ferry Service to Portaferry is very frequent, usually about 15 minutes.
If you have a couple of hours to spend in Downpatrick, visit the Cathedral and view Saint Patricks grave in the Cemetry close by, also the grave of Thomas Russell, ["the man from god-knows -where"] who was hanged at Downpatrick Jail in 1803 for being a leader of the United Irishmen. The poem and song have been discussed on a mudcat thread.
The journey is a doddle, don`t be put off by what looks like, but isn`t, a complicated route. Ard Mhacha.