This seems to be a question about English, not Irish. In English you usually use the article “a” for countable nouns, such as apples, pens, or books. Uncountable nouns auch as sugar, water, or time do not take the indefinite article. Note that they also do not normally form a plural.
Irish doesn't have a word for "have", it relies on a combination of the verb bí (tá in the present tense) and the preposition ag. Tá leabhar ag Pól - "Paul has a book".
In Irish, when the object of a preposition is a pronoun, they are combined - ag + mé produces agam, ag + tú produces agat, etc. You can look up the full set of "prepositional pronouns" for any preposition by looking up the preposition on teanglann.ie, and checking the Grammar tab. The list for the preposition ag is here.