To enter Ireland, both the Republic and the North, you will need a passport valid for at least six months after your proposed date of departure.
Visitors to Ireland are generally exempt from any visa requirements if they hold a valid Canadian or US passport. If you plan to enter Ireland as a tourist and for up to three months only (Northern Ireland up to six months) you will not need to apply for a visa. Similar visa waivers are in place for business travellers and students.
Note that if you plan to take up employment either in the Republic of Ireland or Northern Ireland you will have to apply for a Working Visa first. Some exceptions are in place for bona-fide students.
There are no required vaccinations for entry into Ireland.
Attention - cross-border travel between Ireland and Northern Ireland is generally easy and involves no passport control. If you have a visa for the Republic of Ireland, you are NOT automatically entitled to visit Northern Ireland and vice versa. Take care!