Strictly speaking, it's not a triphthong - the "i" isn't pronounced, it's just there to balance the é on the other side of the consonant. The é is clearly pronounced.
It's an example of the leathan le leathan, caol le caol rule that is fundamental for Irish spelling.
Without over-complicating it (because you are still a beginner), "an" is used for singular nouns, and "na" is used for plural nouns.
- An buachaill -> The boy
- Na buachaillí -> The boys
The rules change when you get to the genitive case, but no need to worry about that until you get there ;)