When siad is used in a phrase, does it have to refer a specific group of people, rather than being a translation for the "they" that is just referring to people in general/an unspecified group?
It isn't quite that simple. For a non-specific "they", the saorbhriathar is often preferred, so the NEID suggests:
deirtear nach féidir rithim a fhoghlaim - "they say you can't learn rhythm"
ní fhéadfaí é a dhéanamh, a dúradh - "it couldn't be done, they said"
for the "people in general" meaning of "they".
Because English doesn't have a direct analog of the saorbhriathar, you need to be careful about using or interpreting siad as a general "they".
Thanks. I'd like to check my understanding: The saorbhriathar, also known as autonomous or impersonal, form of a verb in Irish is similar to the passive form in English, with the main exception that it cannot take a subject (which might make a useful update on https://www.duolingo.com/skill/ga/Passive/tips-and-notes). So the best translation of an autonomous verb might be the passive, but it also might be a circumlocution like "they do" or "one does".
With regard to using siad to mean people in general, in some cases it works, in other cases it would sound unnatural or confusing, and probably the only way to know is through experience.
Is that about right?