In 1871 they are in Stranraer:
Volume 899 ED 9 Page 4
11 Back Rampart
John, 46, labourer, b. Ireland
Jane, 37, b. Crosshill, Ayreshire
John, 13, scholar, b. Stoneykirk
Jane, 10, hand loomer ? b. Inch
Alexander, 8, scholar b. Stoneykirk
Robert, 5, scholar, b. Stoneykirk
Francis, 3, b. Stoneykirk
Mary, 4 mths, b. Leswalt
No luck on the 1861 as yet

Apart from the Twynholm one, the only other John Williamson, b. Ireland, on the 1851 is an ag lab in Mochrum. Wigtownshire, aged 25.
Gadget