I looked at a route today and if we were circumnavigating (and I didn't actully think of doing so till today) it would be an 82nm round trip. We would go round the island on the way there simply to make the trip home the shortest as it is likely to be later in the day.
The leg out and round the island anti-clockwise to Douglas works at around 52nm and another 32 miles from Douglas direct back to Whithorn with another couple of miles for the harbour entrances/exits.
We would need to leave at around 08:15 to make it or 11am at Douglas, weather permitting. Certainly something to think about