What would the name be in this situation then? There isn't any other name for the GNR Main Line in its entirety from Connolly to Great Victoria Street (to Grand Central once that opens). I disagree with the fixme you propose at https://www.openstreetmap.org/way/1272406206 :
Up/Down Line was previously in use as the name, but those are names for tracks rather than the line as a whole and railway:track_ref is the more appropriate tag. "Down Line" isn't a unique term for this specific railway: it's a general railway term. In this case, the down line has trains travelling to Belfast, while the up line has trains travelling towards Dublin. See the West Coast Main Line near London as an example of what I was basing things on.
(https://www.openstreetmap.org/way/224160441)
This is unrelated to the discussion of names, but the Great Northern Railway lasted until 1958, well past the 1920s.