A revival of this blog to add a link to a new online tool…
Very barebones, but it should work for converting many coordinates in some columnar text format (eg CSV)
Can convert to/from OSGB easting/northings and Grid References, dito for the Irish Grid, and WGS84 decimal lat/long.
All done in javascript, so should be reasonably quick, and no formal limit to the number of conversions it can do – although of course it will be slow to do a really large number.