I stand to be corrected but I'd understood it to be a metaphor for a bottle of liquor/poitín. Porter would be the immediate assumption from a word like dubh but the poem itself is from the mid 1700s, before stout porter was actually even on tap in Ireland. Porter was first developed in England in the early-mid 1700s, with urban Irish breweries like Guinness only switching to it from ale in the 1770s-1820s as it became popular first in cities and then later in rural areas in Ireland. May be entirely unrelated but the Scots Gaelic equivalent to poitín is called poit dubh?