Let's jump right in!
Sometimes the ballot says, "Vote for 5" among a long list of candidates. It really means vote for up to five. You can vote for three, or even just one.
Let's suppose you want to elect your favorite three candidate when there are 5 offices to be filled and you can vote for "up to 5". One theory is that "you have 5 votes so you should use them". You vote for your favorite three, and then pick two more at random, or because they have Irish names, or because they are women, or because they don't have Irish names.
Then the results come in; the winners include one of your three favorites and two candidates you voted for at random, not because you thought they were good candidates. Your two other favorites finish sixth and seventh. They lose. You lose.
One vote rarely matters but sometimes a few votes make the difference. So if you had withheld your fourth and fifth vote from the Irishmen who finished fourth and fifth and several of your neighbors also did likewise, the Irishmen drop below your three favorites and all three of Your Candidates Win.
Your candidates win; you win. Happy, happy. Good job.
Why the name bullet voting?
It has nothing to do with firearms. The original use was when you voted for one and only one candidate when you were entitled to vote for multiple candidates, often three. We use the concept here in the more general case (vote for three, not five) without changing the name.