Here is the current irregex 0.9.8 behavior on searching for an empty regex, as a building block for later.
$ guile --no-auto-compile -l irregex.scm
GNU Guile 2.2.3
[...]
scheme@(guile-user)> (irregex-replace/all "" "aaa" "*")
$1 = "*a*a*a"
scheme@(guile-user)> (irregex-replace/all "" "\n\n\n" "*")
$2 = "*\n*\n*\n"
scheme@(guile-user)> There is no match at the very end of the input. This may simply be the behavior desired by the author. If it is not desired, the loop end condition of irregex-fold/fast can be made strict.
https://github.com/ashinn/irregex/blob/ac27338c5b490d19624c30d787c78bbfa45e1f11/irregex.scm#L3807