Bacon and potato bread is a Northern Irish sandwich where the bread is not bread in the wheat-loaf sense at all. Potato bread, the soda-farl-adjacent griddle flatbread of the Ulster fry made from mashed potato, flour, and a little fat, is cooked flat on a hot pan or griddle until it is soft inside with a faintly crisp surface. Here it is fried, usually in the bacon fat, and used as the carrier itself. That is the defining fact: this is not bacon in a roll but bacon between or folded in slabs of fried potato bread, a starch-on-starch construction with no leavened crumb anywhere in it. The flavour follows from the carrier, the potato bread's mild, potatoey, slightly nutty fried surface standing in for the wheat note a roll would bring.
The build is governed by what fried potato bread does and does not do. It has very little structural give and almost no absorbency compared with a soft roll, so it does not soak up bacon fat the way a bap is designed to; instead it carries the fat on its fried surface, which is why it is cooked in that fat in the first place, picking up salt and colour as it goes. The bacon is rendered and crisped as usual, but the texture logic is reversed from a normal butty: rather than crisp bacon against a soft, yielding crumb, this is crisp bacon against a denser, griddled potato slab, two firm elements rather than one firm and one soft. There is no butter step, because the frying fat has already done the job butter does in a roll, bridging the bacon to the carrier and seasoning the contact face. It is eaten hot off the pan as part of, or as a one-handed version of, the Ulster fry, and it does not survive sitting the way a sealed buttered roll does, because there is no waterproofed crumb holding it together.
The variations are the rest of the Ulster fry this is cut from and the regional carriers that parallel it. A fried egg with a runny yolk is the most common addition, the yolk doing the lubricating work the missing butter would; soda farl is the other Northern Irish griddle carrier for the same bacon; the Scottish tattie scone is the closest relative across the water, a different region's fried potato base for the identical filling. Each of those deserves its own article rather than being crowded in here.