"That boy runs to her."
Translation:Ritheann an buachaill sin chuici.
I'm not sure that I've come across "sin" yet, but I'm gathering that it refers to the demonstrative "that one there"?? Is that correct? (were it not for the exercise for me being "select/check the appropriate translation" I would not have known otherwise how to translate "That" boy)
Is "sin" applicable/generalizable to all numbers (sing/pl), persons (1st/2nd/3rd), and genders, or only specific to a certain subset?
Yeah, basically. To say "that [thing]" as Gaeilge, you put "sin" (shin) after the noun, rather than replacing the article with it like you would in English (so I guess it's like saying "the thing there"). The equivalent for "this" is "seo" (shuh). Afaicr, they don't change at all for different cases.
E.g.'s: An buachaill sin = That boy;
Sin (é) an buachaill = That's the boy;
Na buachaillí sin = Those boys;
Cad (atá) é sin? = What's that?;
Seo é mo chat = This is my cat (kind of introducing it, I think).
You might see something like "Sé mo chat", which is a contraction of "seo + é" or "is + ea"... Actually know I think of it, it might just be "is ea"... hmm. ('Sé mise an duine níos sine i mo chlann... I always felt like that had a nice rhythm to it :P).
There might be a modified form of the word for emphasis (like "that boy there"), the way you have with "mé -> mise", but I'm probably just confusing it with "sibhse" >.>