Individual countries follow their own rationales in establishing their criteria for citizenship. Each country has different requirements for citizenship, as well as different policies regarding dual citizenship. These laws sometimes leave gaps where the acquisition of other citizenships does not render the original citizenship invalid, creating a possible situation for an individual to hold two or more nationalities. Here are common reasons to bestow citizenship:
At least one parent is a citizen (jus sanguinis).
The person was born on the country's territory (jus soli)
The person marries a person holding the citizenship (jure matrimonii).
The person becomes naturalized.
The person was adopted from another country as a minor and at least one adoptive parent is a citizen.
The person, a few countries, makes a substantial monetary investment: Austria,Cyprus, Dominica and St. Kitts & Nevis.
Once citizenship is bestowed, the bestowing country may or may not consider a voluntary renunciation of that citizenship to be valid. In the case of naturalization, some countries require applicants for naturalization to renounce their former citizenship. However, this renunciation may not be recognized by the bestowing country. Technically, the person in question may still possess both citizenships.
For example, the US requires applicants for naturalization to renounce all prior allegiance to any other nation or sovereignty as part of the naturalization ceremony.In the case of a British citizen, however, the UK honors renunciation of citizenship only if done with competent UK authorities.Consequently, British citizens naturalized in the US remain British citizens in the eyes of the British government even after they renounce British allegiance to the satisfaction of U.S. authorities.
The Republic of Ireland frames its citizenship laws as relating to "the island of Ireland", thereby extending them to Northern Ireland, which is part of the United Kingdom. Therefore, anyone born in Northern Ireland who meets the requirements for being an Irish citizen through birth on the island of Ireland (or a child born outside of Ireland but with a qualifying parent) may exercise an entitlement to Irish citizenship by acting in such a way that only an Irish citizen is entitled to do (such as applying for an Irish passport). Conversely, that such a person has not acted in this way does not necessarily mean that they are not an Irish citizen. See Irish nationality law and British nationality law. People born in Northern Ireland are British citizens on the same basis as people born elsewhere in the United Kingdom. People born in Northern Ireland may choose to hold a British Passport, an Irish Passport, or both.