I thought Shelbourne were always Rovers' traditional rivals before they decamped to Dublin 9.
Presumably the rivalry from Drumcondra was due to them being a Northside club, ditto Bohs. Bohs had the Northside sewn up really when Drumcondra melted into Home Farm -- and I doubt if they had great relations with Rovers up to that point anyway.
It's mostly due to Rovers being *****s, though, obviously.
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