The Ulster fry sandwich is a Northern Irish cooked breakfast loaded into bread, and what defines it is that the bread is part of the fry rather than only a wrapper around it. A full Ulster fry runs bacon, sausage, and egg alongside soda farl and potato bread that have been fried in the pan, and the sandwich keeps that logic: the griddle breads come along as filling, so the build is starch wrapped around more starch with the salt and fat of the fry running through the middle. That doubling is the identity. A bacon roll is a fried breakfast carried by bread; the Ulster fry sandwich is a fried breakfast that brought its own fried bread inside, which is exactly why it reads as Northern Irish before it reads as a breakfast roll.
The craft is grease and structure across a lot of cooked components. Each element is fried to render and crisp it, and a soda farl or a slice of potato bread that has been through the same pan is already saturated with fat, so the outer roll has to be soft and absorbent enough to take more grease without going to paste and sturdy enough to hold a heavy, multi-part filling together for the walk. The sausage is split lengthways so it lies flat and the stack does not roll apart; the bacon is crisped at the edge so it has bite against soft fried bread; the egg is the moisture problem, run hard if the sandwich has to travel and soft if it is eaten standing over a plate. Butter on the roll is still the bridge between the salt of the meat and the wheat, and brown sauce applied inside is the standard sharp counter to a build that is otherwise rich the whole way through.
The variations are the rest of the Northern Irish breakfast folded around the same fried-bread core. Adding white pudding or black pudding deepens it; a soda farl used as the outer bread itself rather than as filling makes the doubling literal; vegetable roll or a slice of fried tomato cuts the richness; the plain bacon-and-egg-on-soda reading strips it back to the essentials. Each deserves its own article rather than being crowded in here.