Deborah asked:

> An Bíobla Naofa gives "rúndiamhara" for "mysteries" (1 Corantaigh 4:1),
> and Am Bìoball gives "rùintean-diamhair" (same verse). When and why
> did "diamhara / diamhair" get tacked on, and what is the larger
> semantic range of "rún" in OI?

The semantic range of "rún" in OI was actually fairly restricted.
If you look at the entry in FGB (aka Ó Dónaill's), meanings 1 and
2 go back to OI.  Number 3 develops in MI, and 5 is limited to
EMI.  (Number 4 strikes me as entirely modern, but that's just
an assumption.)

The entries for "rúndiamhair" in DIL date from the early 17th
century or later.  By that time the semantic ranger of "rún"
was apparently felt to be too great to be able to rely on the
unmodified word to convey the meaning "(religious) mystery".