According to The Sportsman in Ireland by “A Cosmopolite”, originally published in 1840, the “Sportsman’s breakfast” of the time was “First, a large bowl of new milk which instantly disappeared; then a liberal allowance of cold salmon soaked in vinegar — a very common dish this…and a bottle of port wine.”
Because salmon was so plentiful in Ireland until recent times, it was often preserved, not only by smoking but also by salting and pickling — and the salted and pickled fish was an important item of export, especially in the north of the country, for instance from the port of Ballyshannon in Donegal. I found this recipe (minus the quantities) in Mrs. A. W. Baker’s Cookery Book, Vol. 1, a manuscript dating from the early 19th century, from Ballytobin, County Kilkinney. Mrs. Baker notes that her pickled salmon “will keep three months in cold weather”.
Adapted from "The Country Cooking of Ireland" by Colman Andrews
Put 1 quart water into a large pot, then add salt, peppercorns, mace, nutmeg, and allspice. Bring to a boil over high heat, then turn off heat and let rest for 10 minutes. Repeat process twice more, then add salmon to pot and bring to a gentle boil over medium high heat. Simmer for about 5 minutes, then carefully lift fish from pot with a slotted spatula, being careful not to let fish break apart. Set aside and allow to cool.
Strain poaching liquid through a fine strainer or cheesecloth into a medium bowl, stir in vinegar and set aside to cool.
Put bay leaves in 1 layer on the bottom of a glass or earthenware crock or baking dish with a cover, then lay salmon pieces side by side on top of them. Pour the cooled poaching liquid over salmon, making sure that it covers the fish entirely (add more water and vinegar is it doesn’t).
Cover and refrigerate for at least 3 days and as long as a week, turning fish once a day. Bring to room temperature and serve with brown soda bread and unsalted butter.