"My parents used to tell those stories when I was young."
Translation:D'insíodh mo thuismitheoirí na scéalta sin nuair a bhí mé óg.
I'm a bit confused about when to use "d'insíodh" and when to use "d'insídís." The Teanglann site (http://www.teanglann.ie/en/gram/inis) listed both as possibilities for the third person plural. Does the -ídís version have the pronoun built in? Do you have to use the -íodh version when a noun is the subject?
Yes, the -ídís version has the pronoun built-in. If there's an explicit noun, you use the -íodh form (or if you say d'insíodh siad).
It doesn't accept '...nuair a bhíonn mé óg' and I suppose there's a nuance here. Comparing with the proverb, 'Nuair a bhíonn an cat amuigh...' I would guess the habitual tense means something like 'whenever' as opposed to a continuous state?
nuair a bhíonn an cat - "when the cat is" (does be)
nuair a bhí mé óg - "when I was young"
The séimhiú is caused by a, it's not a past-tense marker.