Very comprehensive and informative list Darrman, thanks for putting it together.
Just to add to it, the bridge numbers may not represent the actual number of bridges/culverts/underpasses on that section. For example, you can have OBC274 (an overbridge) then UBC274A, UBC274B and UBC274C - all culverts after OBC274 in the high mileage direction.
The Limerick Junction loop line appears to be undesignated as you say, but there are no structures or level crossings on this stretch. The mileposts follow the Heuston to Cork numbering but are designated LN (Limerick North) to LW (Limerick West).