Francine Nicholson wrote:

> What word would be translated as "trance"?

The word that comes closest may be "nél", which is in
the Dúil Bélrai, although in both early and modern usage
a "nél" is generally something one falls into, like a
stupor or a faint, rather than an altered mental state
that one actively cultivates.  There is, however, this
example in DIL:

   dochuaidh a n-eaxdasis [= extasis] .i. a néul

   = went into an ecstasy, i.e. into a spiritual "nél"

This is from Mac Aingil's "Scathan shacramuinte na
haithridhe", from the year 1618.