whitrope69
If you run searches on Flickr they will throw up various pictures of freight workings in the 1980s and early 1990s - try searching for Stranraer freight and Stranraer Speedlink.
There is a bit of discussion and some pictures in this thread on RMWeb
http://www.rmweb.co.uk/community/index.php?/topic/18401-stockton-haulage-depot-stranraer/
A few freight-related images here too
https://www.railscot.co.uk/search/index.html?textfield=stranraer#
The Speedlink services in the 1980s tended to originate at Tees Yard, as there was a lot of steel traffic from British Steel at Redcar to Northern Ireland, which was the core traffic for the Speedlink services - and was why Stockton Haulage got involved, the firm being from Teesside. Their Stranraer depot opened in 1975 or 76 on the site of the carnage sidings built during the Second World War. Other wagons with various kinds of traffic were attached and dropped at Ayr Falkland Yard, Carlisle Kingmoor and Tyne Yard as required. Bunker oil for the ferries was also carried for many years.
One bit of outbound traffic I recall from the late 1970s was feta cheese from the Stranraer creamery which was taken to Turkey in Interfrigo insulated ferry vans, which I assume were brought empty to Stranraer after bringing other traffic into the UK, though I think this was just a short-lived trial.
A train only carrying steel operated from Teesside to Stranraer for a while after the end of Speedlink, but it stopped in, I think, 1994.
Speedlink services were operated over the years by 26s, 27s, 37s and 47s, though pairs of 20s even got a look-in sometimes. See
http://www.geograph.org.uk/photo/3268480
Class 40s seem to have pretty much disappeared once vacuum-braked freight via Girvan ended in the late 1970s.
For many years there were also daily parcels van trains from and to London Euston.
Freights get a mention along the way in Girvan & Portpatrick Railway and The Portpatrick & Wigtownshire Railways, both by CEJ Fryer (Oakwood Press), including longstanding daily overnight trains between Stranraer and College Yard in Glasgow.
Branches & Byways Southwest Scotland and the Border Counties by Robert Robotham (OPC) has a lot of information about services between Dumfries and Stranraer and pictures of freight trains from the 1950s and 1960s along that route. Until the late 1940s there were dedicated milk and fish trains to Dumfries and Carlisle.
I would have to take issue with the suggestion the ship ran once a day. Perhaps in the early years after the line from Dumfries opened but there were morning and evening sailings both ways between Stranraer and Larne from the late 19th century.
There were some facilities at the harbour provided long before the days of the lorry park - notably a large cattle dock for transfer of livestock, which was an important traffic for many years. There were also a few sidings at the town end of the pier, though not sure whether there was a loading dock next to any of them. This was where the ramp to unload new cars from Speedlink services was later built.
I would endorse the recommendations of David L Smith's books about the railways in the South West of Scotland - whatever your specific interests they are a good read.