Day trip to Galloway Distillery From Northern Ireland - surrounding area recommendations

That's a brand new distillery.

Very unlikely to be taxis as almost everyone uses their own car or the buses to Glasgow.

So you will need to pre-order one.

Nearby points of interest Wigtown (book town and the somewhat grisly Covenanter History)

Garlieston for the WW2 mulberry harbours.

Whithorn and the Isle of Whithorn for one of the cradles of Christianity in Scotland.

Given the expense of the taxis you would almost certainly be better off bringing your own car over.

For me places to eat the Community Cafe or the pub at Isle of Whithorn.

If you look at Google Street View, there's a parking area for taxis https://maps.app.goo.gl/AmKGUdETuDfjbNkb6

Also, the ferry port indicates taxis are available https://www.stenaline.co.uk/routes/cairnryan-belfast/cairnryan-port#tabs-c4ef987d0f-item-6dd2c2e545-tab

But I would book one so it's there when you get off the ferry.

Is this a day trip from Belfast to Scotland and back to Belfast?

Have just looked up the taxi rates- £5.50 for the first 1 1/2 miles, then £1 for each extra 1/2 mile - so at 28 miles that is £61.50 each way + any dead mileage.
The Isle of Whithorn is another 20 miles, and the Isle to Cairnryan is 37 miles.

And that is 3 taxi rides, around at least £170 in fares. There are bus services south to Stranraer, but they don't connect well with the ferries.

So a pre booked deal would be best to try to get.

Having used the port a number of times I can't remember there being taxis waiting there in case of trade, as opposed to pre booked.