While I agree that people understand "have (a piece of information)" as equivalent to "know (a piece of information)", I wouldn't consider using "know" instead of "have" to be a "more accurate" translation, just a possible alternative way to say the same thing. "Do you have John's phone number?", "Do you have the date for that party yet?", "Do you have the name of a good mechanic?", could all be said with "know" instead of "have", but the Irish for "I don't know the population" is Níl an daonra ar eolas agam, or Níl a fhios agam an daonra, so there really isn't any need to translate Níl an daonra agam that way.
daonra na hÉireann is a genitive phrase (Éireann is the genitive of Éire). In Irish, the definite article in a genitive phrase makes the whole phrase definite, so the English for daonra na hÉireann is "the population of Ireland". You don't put an before daonra na hÉireann, because that would means "the the population of Ireland".