OK, here's the definitive answer, with info straight from our developers.
Normally GridInquest is not meant to be used for Irish to British grid conversions, but I mentioned that it's a fairly common practice and so our developers will fix it so that an Irish -> British conversion doesn't cause an out of range error in the GridInquest transformer. We're hoping for this to make it into FME2017.1
So those points we transformed. That will work and not cause an error.
Under the covers I think it's a two-step process where we convert Irish Grid to ETRS89, using "the official polynomial transformation parameters", then ETRS89 to OSTN2015 using the NTv2 grid. But you wouldn't really need to know that.
So - in short - try the GridInquest transformers in 2017.1 when it is released.