Both have there pro's and con's. A guided tour will give you the same experience that the average tourist gets. You'll see the main sites, but will probably never see the 'real' Ireland. It will however be easy and less planning on your part.
If your the type that can laugh at the "i think we should've turned left back there", then getting off the beaten path can help make it a honeymoon to remember.
Check out About.com's
Irleand page. Pay special attention to the links on the left hand side of the page. You'll see essentials, Offers, and Topics. You can get a ton of travel information. It will also help you decide on which side of Ireland you really want to see. Historic, or 'Happening'? Dublin is considered one of Europe's top party towns. So if you're into the nightlife, that would be a great place to start. If you're looking for the calm that a great old castle and the rolling bluffs of the green hillside, there are tons of choices too. Make note of the weather during the time of year you plan on going. It may affect the activites you plan on doing.
Bottom line: Do a ton of research. Start online and search 'ireland' on the major search engines (Google, Yahoo, MSN,....maybe Ask.com) and work from there. There are also great informational sites like about.com and wikipedia.com. They have good general information.
Hope this helps. Have fun...Ireland looks like a fun place to visit. Hope I have the pleasure someday.