For centuries it was just a hole in the ground in the vicinity of the house. For olfactory reason it was not too close. Then someone decided that it would be nice to have a roof over the hole so you do have to stand in the rain or snow. Of course walls were soon added to add privacy and also limit exposure to the elements. Now you have a “small house” near the house. Otherwise called the “out house.” In door plumbing made it reasonable to bring the facility inside the house. But in Gaelic it appears the name stayed with the function of the room. The little house.