The word gulpin hasn't, as far as I know, travelled south from Ulster. It is from the dialect gulp, the young of any animal in its softest and tenderest state, a word found in Scotland and in Northumberland, Lincolnshire, East Anglia and Hampshire. Hence gulpin, a young child, and figuratively, a gullible man, a word of Anne Gallagher's mother, who moved south from Creeslough in Donegal many years ago to set up home in Dublin.
Gulp comes from the verb meaning to swallow, which has a cognate in the Dutch gulpen, to guzzle. It has been in English for a long time; Langland has y- gulpid and y-golped in Piers Plowman in the 14th century. Gulpin is probably from the compound gulp in; young animals are good at it. It came to mean a credulous person. In British naval circles in the 19th century, a gulpin was a marine; Notes and Queries, one of academia's most venerable journals, said in 1867 "a marine was called a gulpin by the sailors, that is a person who would swallow anything told to him".
Walter Scott, in St Ronan, has the word galopin, a minor servant in a big house, which an American academic recently glossed as being a variant of gulpin. It isn't. It is from French gallopins "under cooks or scullions in monasteries", according to Cotgrave's A Dictionarie of the French and English Tongues, published in 1611.
Martin Vernon of Killard, Doonbeg, Co Clare, wrote inquiring about the phrase to pay through the nose, to pay an exorbitant price. The best I can do is quote from Nigel Rees's Dictionary of Phrase and Illusion. According to Rees: "one possible explanation lies in the `nose' tax levied upon the Irish by the Danes in the 9th century. Those who did not pay had their noses slit." You can take that or leave it.
This was a cant phrase in the 18th century, first used in literature by Andrew Marvell in a tract against popery, written in 1672: "Made them pay for it most unconscionably, and through the nose". Fanny Burney has the phrase in Cecilia a century later: "She knows nothing of business, and is made to pay for every thing through the nose." A real gulpin, Fanny's woman, I was tempted to write, until I remembered that in the course of a long association with Donegal, I have never heard gulpin used of a grown woman.