U.S. Passport holders DO NOT require visa to enter Ireland. Non U.S. citizens should check to see if whether they require a visa.
The following Citizens listed below DO NOT require a visa to enter Ireland:

Andora, Antigua & Bermuda, Argentina, Australia, Austria, Bahamas, Barbados, Belgium, Belize, Bolivia, Botswana, Brazil, Brunei, Canada, Chile, Costa Rica, Croatia, Cyprus, Czech Republic, Denmark, Dominica, El Salvador, Estonia, Fiji, Finland, France, Germany, Greece, Grenada, Guatemala, Guyana, Honduras, Hong Kong (SAR), Hungary, Iceland, Israel, Italy, Japan, Kiribati, Korea (Rep. of South), Latvia, Lesotho, Liechtenstein, Lithuania, Luxemburg, Macau (SAR), Malawi, Malaysia, Malta, Mauritius, Mexico, Monaco, Nauru, Netherlands, New Zealand, Nicaragua, Norway, Panama, Poland, Portugal, Saint Kitts & Nevis, Saint Lucia, Saint Vincent & the Grenadines, San Marino, Seychelles, Singapore, Slovenia, Solomon Islands, South Africa, Spain, Swaziland, Sweden, Switzerland, Tonga, Trinidad & Tobago, Tuvalu, United States of America, U.K. & Colonies, Uruguay, Vanuatu, Vatican City, Venezuela, Western Samoa.

Jurisdiction:
We can only process your visa if you live in either the New York or Washington jurisdiction.
New York: New York, Connecticut, New Jersey, Pennsylvania, Delaware, West Virginia, North Carolina, South Carolina, Georgia, Florida
Washington: Washington DC, Virginia, Maryland

Requirements for obtaining a Business & Tourist Visas:

• A passport valid for a minimum of six months from the date of intended arrival in Ireland (with at least one blank page available)
• One Visa Application Form, filled out, dated and signed by the applicant
• Two recent passport photo
• A copy of round-trip ticket or a booked itinerary showing travel to and from Ireland
• A copy of your US Alien Resident Card for foreign residents and/or a valid visa or document to return from Ireland to the US or to another country
• Letter from employer
• Proof of Financial Resources - A recent Bank Statement of your account
• Submit a copy of your Hotel Confirmation or a Letter of Invitation if you're staying with friends or relatives