In America, the government doesn’t technically have the right to know where you live. You don’t automatically get to vote without registering because the government doesn’t necessarily know where you live. You have to give your address to get a driver’s license, so often that government office also does voter registration.
Before our elections, we have primaries. That’s where each party picks who the candidates are from that party, but they are a state-run event. Technically a party doesn’t need to do this, they could just submit their candidate to the final election. Every state does it a little differently, but in many states, you need to declare which party you are a part of in order to take part in the decision for who that party is sending to the election. In some states, it’s optional, and when you go to vote in a primary, you tell them which party you want to select the candidate for, and they give you that ballot. Other states are weird and do what’s called “caucusing”, where everyone from a particular party within that voting district meets up, and they try to come to a decision on which candidate to send forward. In those states, it’s not a blind vote, but you essentially get a sort of instant runoff, at least in my understanding.
It looks like in Ireland, since you have multiple parties, you have more options at your elections, so you don’t have to help the parties pick their candidates?