Sometimes it can be pretty confusing searching for Irish ancestors, particularly given all of the names which have been used throughout history to describe the mass of islands, just west of continental Europe, of which Ireland is a part. Here are some geographical and political designations to make matters more clear.
The BRITISH ISLES: - this is a GEOGRAPHICAL designation. It includes the two large islands on which you find the countries of England, Scotland, Wales, Ireland, and the State of Northern Ireland. In addition, around six thousand smaller islands fall under this geographical designation, including the Isle of Man, and the Channel Islands.
Since the early part of the 21st century, in some circles there has been a movement away from use of the geographical term 'British Isles' toward terms such as 'Anglo-Celtic Isles', because of the thinking that British Isles is a politically charged phrase.
This geographical designation differs considerably from the political one. A geographical designation is not about who rules over whom; it is about proximity not politics.
The UNITED KINGDOM: - this is a POLITICAL designation and includes the countries which make up what is formally referred to as the 'unitary state' of the United Kingdom. To call this a unitary state simply means that the member countries are governed under a constitutional monarchy and a parliamentary system based at Westminster in London, England. The formal name for this entity is the 'United Kingdom of Britain and Northern Ireland', but it is commonly known as the United Kingdom or the UK.
This political designation includes the following:
State of Northern Ireland
Although still part of the parliament at Westminster, Scotland, Wales, and Northern Ireland are each currently run under their own 'devolved' administration, which operates out of their respective capital city. In effect, this translates to them having increased say than they had in the past about what goes on in their own countries.
The currency used in the United Kingdom is the British Sterling Pound (GBP).
Although it is incorrect to do so, some persons use the geographical and political designations interchangeably. This can lead them to mistakenly refer to citizens of the Republic of Ireland as British. Today, it is entirely incorrect to refer to citizens of the Republic of Ireland as British.
The REPUBLIC OF IRELAND:
In 1922, after the Irish War of Independence, the 26 counties of the Republic of Ireland became Saorstát Éireann, the Irish Free State. This means in some respects it was still connected to Britain politically, but was governed by its own parliament, instead of the English parliament at Westminster.
In 1937 with the drafting of the Constitution of Ireland, the name of the Irish Free State was officially declared as ÉIRE, in English, IRELAND.
In 1949, the remaining powers of the British monarch were removed, all ties to the constitutional monarchy of Britain were severed, and Ireland declared itself the REPUBLIC OF IRELAND.
The Constitution of Ireland states, "[t]he name of the State is Éire, or, in the English language, Ireland". Irish statute law of 1948 offers clarification. It states that Poblacht na hÉireann, in the English language, the Republic of Ireland, is "the description of the State" not its official name.
Whether you call it Éire, Ireland, or the Republic of Ireland, this is an entirely separate country from the UK. Its citizens are subject only to Irish laws and conventions. It is a member of the European economic community, so its currency is the Euro, not the British Sterling Pound.
In terms of your research, depending on the time period you are searching, you may come across these various designations on documents about your ancestors.
For example, if you are searching for an Irish ancestor in the time period pre-1922, 'British' or 'English' is usually the designation given for citizenship or nationality, with 'Irish' given as the designation for 'Race'. From 1922, and until Ireland became a republic, 'Irish Free State' may be indicated as the country of origin for your ancestor, on such documents as ships' passenger lists. This simply means the person in question hailed from one of the 26 counties in what is now called the Republic of Ireland.