A scalp was a pauper's hut dug into the Irish loam. They are most often referred to in writings and retellings of the 1800s, especially the Irish potato famine, but they are a fairly straightforward idea, and surely go back thousands of years.
To construct a scalp, one digs out loam, stacks it up around the side of the excavated hole, and then roofs over the pit in whatever manner one can -- sometimes thatch, sometimes blocks of loam supported by branches or crude beams. If constructed into the side of a hill, a scalp might have something approaching a door and windows on the downhill side; in flatter areas, it might look much more like an a cave or animal burrow dug in a hillock.
These were the homes of the landless laborer; some were described as ditches with a roof lain over them, and they might well be not much more than this; they would contain a fire pit, enough room to sit, sleep and cook, and that might well be it; wooden furniture was a luxury. They might also house the family's pigs, if any (a great way to turn food waste into meat or money).
A scalp that raised up a bit more above the ground, clearly recognizable as a human construction, might be called a 'scalpeen'; this appears to be constructed from the Gaelic suffix –ín, a diminutive, but in this case perhaps used as a pejorative. The scalpeens tended to use debris from ruined houses, sometimes being built within the rubble of a house pulled down by an angry landlord.
It should be noted that peat blocks are not a terrible building material, and small cottages made of peat blocks with proper roofs, windows, doors, and often a wall or two made of wood or stone would generally not be called a scalp; a scalp was a hopefully temporary, crudely constructed home built without the landowner's permission. However, during times of famine and societal collapse, boundaries blur, and what exactly counts as a scalp vs. a scalpeen vs. a peasant's hut might be a matter or personal opinion rather than fact.