This might be a matter of English dialects. I’d consider this sense of “interest” to be countable, and thus I’d tend to use “an interest”. May I ask which variety of English you speak?
Would you consider “He has interest in new people” and “He has an interest in new people” to be completely identical in meaning, or do you discern any difference in meaning between them?
I'm midwestern American and I'd say the two are pretty interchangeable to me. If I tried to differentiate in that sentence, I'd say "an interest" might be something he'd want to learn (study) about and "interest" might be something passingly entertaining (fun/a good conversation?). But really, I wouldn't blink at "an" or no "an" being used either way.
I agree that both variants are readily comprehensible, but “an interest” means that “interest” is countable, and “interest” without “an” means that it’s a mass noun — compare “She has a cheese” vs. “She has cheese”.
So, how would you write an uncountable/mass noun in Irish differently than a countable one? (And thanks for continuing this conversation so long after it started!)
They wouldn’t be differentiated in Irish, just as a singular noun vs. a mass noun isn’t differentiated in English (other than with an indefinite article being used in English, which isn’t an option in Irish).
Spéis is more emotional: a fondness for. More internal. The heart's interest. Suim, which also means a sum/value is more practical, calculated, based on the value of things. More external. The brain's interest.