I'm currently working in a database which stores mainland British locations as their Ordinance Survey Northing/Easting values. Can points in Ireland (primarily Northern Ireland) be accurately represented via simply adding to the datum or is the curvature too much?
Technically, yes, depending on how good the algorithm for transverse Mercator is in the software that you're using. The origin of BNG is 2 West, and Northern Ireland extends to 8 West. Six degrees from the central meridian (2 West) is not bad and should work fine for any standard implementation of transverse Mercator.
As mentioned in comments, it just isn't as suited to Ireland and Northern Ireland as the Irish TM coordinate reference system.