Translation:We went to the cave but it was dark.
NOTE: Since "uaimh" is feminine, the "sé" of "bhí sé dorcha" cannot refer to the cave so would seem to refer to an indefinite "it" as in "it's dark out" or "it's getting dark". (Correct?)
Yes. (In the reverse exercise, I’d reported that both sé and sí should be accepted for “it” because the context is ambiguous in the English sentence.)
That would be my opinion of it as well. Not that the cave was dark, but that it was dark outside. (kinda like the sé in tá sé ag cur báistí)
That would seem reasonable. See this example: "Bhí áiteanna ar imeall na coille faoi sholas, ach bhí sí dorcha istigh ina lár" where "sí" would refer to the feminine noun "coill", here in its genitive form. The quote is found in the "An Fhrainc" section of the Irish version of "Tales of our Forests" ("Scéalta faoinár bhforaisí") downloadable here: http://bookshop.europa.eu/en/tales-of-our-forests-pbKF0415081/