Whether you need a passport for Ireland depends entirely on your citizenship:
No passport required. Under the Common Travel Area (CTA) arrangement, citizens of the UK (including Channel Islands and Isle of Man) and the Republic of Ireland can travel freely between the two jurisdictions. However, you must carry a valid photo ID (e.g., driver's license, national ID card) for identification when crossing borders or boarding transport.
A passport is mandatory. Additionally, depending on your nationality, you may require a visa to enter Ireland. Check the Irish Naturalisation and Immigration Service (INIS) website for specific visa requirements based on your citizenship.
Even if not strictly required (for UK/Irish citizens), carrying a passport is often the most universally accepted form of ID and can simplify travel, especially if connecting through other countries.