The story
From Blackrock, a town near Dublin in Ireland.https://web.archive.org/web/20040427211509/http://www.cantrell.ie/our_brands/club.htm See on the rocks (“served with ice”).
Etymology from Wiktionary, CC BY-SA 4.0.
rock shandy
Today it means: In Ireland, a soft drink whose constituents vary regionally, typically made by mixing orange and lemon sodas. Full definition →