Stays based on tourist visas cannot be extended past 90 days under any circumstances.
• Because of an informal agreement between the United Kingdom and Ireland, known as The Common Travel Area, there are no passport controls in effect for UK citizens travelling to Ireland. On arriving in an Irish airport from the UK, however, you will be asked for valid official photo-identification such as a passport or driving licence which shows your nationality. This is to prove you are an Irish, UK, or EU citizen who is entitled to avail of the Common Travel Area arrangements. Immigration controls are mandatory on all inbound flights, selective on ferries, and occasional at the land border crossings.
By plane
The Republic of Ireland is served by 4 international airports, Dublin (IATA: DUB), Shannon (IATA: SNN) in County Clare, Cork (IATA: ORK) and Ireland West, Knock (IATA: NOC) in County Mayo. Dublin, the 8th largest airport in Europe, is by far the largest and most connected airport, with flights to many cities in the US, Canada, the UK, continental Europe and the Middle East. Shannon, close to the city of Limerick, also has flights to the US, Canada, Middle East, the UK and Europe. Cork has flights to most UK destinations and a wide variety of European cities. It is easily accessed from any of the major European hubs, including all of the London airports. Knock Airport has daily scheduled flights to several UK cities, as well as various chartered flights to (mostly) holiday destinations in Europe.
Smaller regional airports that operate domestic and UK services include Donegal (IATA: CFN), Galway (IATA: GWY), Kerry (IATA: KIR), Sligo (IATA: SXL) and Waterford (IATA: WAT).
The City of Derry Airport, and both Belfast airports (both the City and International) are within a relatively short distance from the North/South border, especially the former. (These three airports being located within Northern Ireland)
