To answer the first question, the subject in the sentence is 'the best things about Limerick' (best being an adjective), and 'things' is plural, which is why the verb conjugates in plural. It's not 'people' because that's the predictive nominative of the sentence. And as for the second, yes! You've now used the singular 'thing', and the verb corresponds. The second sentence is actually better because 'the best' implies that there is no better other thing, and so that there is only one best thing.

"The best thing about Limerick is the people"

singular, as the subject is "thing" so the predicate is "is"