Irish grid reference to vice-county conversion
To convert an Irish grid reference (e.g. O2837) to vice-county number (i.e. VCH21). The converter should work with all valid Irish locations, multiple VC numbers will be returned if the area specified spans a boundary.
For British localities there is a separate vice-county conversion tool
It's also worth trying herbariaunited's placename to vice-county converter derrived from the USGIA gazetteer.