Let's see - Dublin Airport to Strangford is 160km, roughly 2 1/2 hours on Irish roads and assuming there's no traffic jam in Newry.
That means the ferry probably isn't an option. The long route 'round takes more than 3 hours from Dublin, so then it doesn't matter whether you land in Belfast or in Dublin :)
Is 20:50 the earliest possible time?
Letty