> "Chi mi e" becomes "chithear e", but what about "èisdidh mi ris"? Is it
> "théid éisdeachd ris"? Are there any other ways to convert this into the
> passive voice?
"Éistear ris" - People will listen him / He will be listened to.
Verbforms such as this (chithear, éistear, thathar, chan eilear)
are called "impersonal" or "autonomous". Even though they are
very often used to translate the English passive voice, they
should not be called "passive", because they are not passive
voice in Gaelic (which has no passive voice). Rather, they are
active voice verbs with no subject, which is something you don't
get in English or in most other languages. In most languages you
have to use a dummy subject in such circumstances ("you" or "people"
in English; "on" in French; "man" in German):
Chithear e gu tric.
You see him often. / People see him often / He is often seen.
On le voit souvent.
Man sieht ihn oft.
And as you said, the English passive can also be expressed by phrasal
constructions in Gaelic:
Théid a fhaicinn gu tric.
Chaidh a fhaicinn gu tric.
Théid éisteachd ris.