I checked the Consular Information Sheet for the United Kingdom and we do not need a visa to enter Northern Ireland. I also found this information on traveling in Northern Ireland:

The political situation in Northern Ireland has dramatically improved since the signing of the Good Friday Agreement in 1998. The Irish Republican Army (IRA) announced on July 28, 2005, that it would end its armed campaign, and agreed to set up a power-sharing government on May 8, 2007. Within Northern Ireland, flash-points for sectarian confrontations still exist, but they are generally removed from areas where tourists congregate. Political demonstrations, however, are well-policed and generally orderly. The potential remains for sporadic incidents of street violence in isolated areas in Northern Ireland during the summer marching season (April to August), with tensions heightened during the month of July, especially around the July 12th public holiday. As a result, American citizens traveling in Northern Ireland could experience delays and disruption.

For the latest security information, Americans traveling abroad should regularly monitor the Department’s Internet web site where the current Travel Warnings and Public Announcements, including the Worldwide Caution Public Announcement, can be found. Recent communications from U.S. Embassy London to the local American citizen community, called Warden Messages, can be found on the embassy’s website.

You may also want to take a look at the U.S. Department of State International Travel website.