My mental block is with the use of the English word "positive" to describe a person. Does the sentence mean the person has a positive outlook (is optimistic), that he/she is positive in his/her opinions, likes, dislikes, etc (is opinionated or inflexible), is positive about doing things his/her own way (is stubborn) or something else?
One could say Is buachaill soirbh é, but I think that that would mean more “He’s a cheerful boy” than “He’s an optimistic boy”. To speak directly of the boy’s attitude, perhaps Tá meon dearfach ag an mbuachaill would suffice — iompar might speak more to the boy’s external behavior than his internal attitude.
For a number of different reasons.
The simplest is that there is no definite article in Is buachaill dearfach é, so the translation can't be "the boy".
The more fundamental reason is that Is buachaill dearfach é is a copula - you are linking the noun buachaill to the pronoun é. "the boy is positive" is not a copular sentence - you are using an adjective ("positive") to describe a noun ("boy"). The Irish for that sentence is Tá an buachaill dearfach.