Is there a differece between

I used to (for a time, but it was limited in time)

I used to (for an indefinte set of cycles)


In Irish 'preterite' is for 'past tense', I know, but the heading had to be catchy

Tá an cheist aisteach agat

If it is *past*, then it cannot be "indefinite set of cycles", past = "the thing is over", so your 1=2

I meant "I used to do that, for 3 years" vs "I used to do that (indefinite)"

Tá tú an caint faoin 'Aimsir Ghnáthchaite', nach bhfuil?
Tá sé le feiceáil ar leathanach 74 i Progress in Irish.

Tá difríocht idir an 'Preterite' simplí agus an Aimsir Ghnáthchaite cosúil leis an difríocht idir 'I did' agus 'I used to do' i mBéarla.

Cosúil le do shampla i mBéarla níl difríocht idir an 'used to' i "I used to do that, for 3 years" vs "I used to do that (indefinite)."
Déanann tú an difríocht leis na focail a thagann ina dhiaidh, cosúil le, mar shampla "for three years."

Rinne mé é - I did it.
Bhí mé á dhéanamh - I was doing that.
Dhéanainn é - I used to do it.
Bhínn á dhéanamh - I used to do it (?!).

I'm not quite sure what the difference between the last two is, probably it's the focus on the duration rather than on the result of the action in the last one.

Nuair a bhí mé i mBaile Átha Cliath shiúlainn go dtí an óstán gach lá.
Nuair a bhí mé i mBÁC bhínn ag siúl cois na Lífe gach lá.

i.e. I used to walk to the hotel - I used to be walking along the Liffey

That's what I think about that, but I'm not quite sure I'm right
Daithí

Bhínn á dhéanamh would be "I used to be doing it". You could use it in contexts like "I used to be doing my homework when my dad would come back home".