I think if you are a US citizen and are planning the legal marriage here in the US, you should be fine just having a symbolic wedidng in Ireland. Here is a web site you can read, and altohugh it doesn't specifically list symbolic weddings, there are phone numbers at the bottom of the link. You can call these numbers and they should be able to provide you wih the relevant information: (call the General Reg. Office-number at the bottom)
Marriage In Ireland - U.S. Embassy Dublin, Ireland