you've mentioned a big clue, in that your Grandfather immigrated from Ireland to the US. and so therefore the ring could have been made in Ireland.
Concerning Ireland country hallmarks, for the UK (England, Scotland, Wales & N. Ireland) & IRELAND: There are mostly 3-5 marks, not necessarily aligned the same and include Maker's mark, Town mark, Standard (Purity) mark, Date Letter, and Duty mark.
The B in a house mark has me stumped. Are you able to send images of the marks?
