I have over seventy books about Ireland and about twenty of those are specifically guidebooks - or guides for the traveler offering information on sites, attractions, accommodations, dining and shopping. Some have maps. Most have photos, but all guidebooks tend to become out of date as prices change and businesses come and go.
Travel guru, Rick Steves makes a good point when he says people are often reluctant to pay the typical cost for a guidebook ($20) when they can get brochures and visitor guides for free. A good guidebook pays for itself over and over by saving the traveler time and money through offering advice and tips on good deals, the best bargains and choices according to budget. (Steves' is one of - if not - THE world's top guidebook writer.)
All good guidebooks will have information about culture, language, transportation, accommodations, shopping, dining, a glossary, maps, directions, and historical facts - with price suggestions based on the traveler's budget. I read extensively in advance when planning my trips, and I include guidebooks in my research. But I take three guidebooks with me on my trips to Ireland. These three have all the qualities mentioned above for good guidebooks, but each of these has something unique that no other guidebook has.
My top three picks for Ireland guidebooks are:
NUMBER 1

Ireland (Country Guide)
by Lonely Planet
I've been a Lonely Planet guidebook fan for years. My primary reason for choosing this as my top pick in COMPREHENSIVENESS. The sites I hit in Ireland aren't always the most popular or famous, so I want a guidebook that will have more of the out-of-the-way sites included. Lonely Planet seldom fails me. This guidebook follows all 32 Counties (includes Northern Ireland), so it completely covers the Island. Additionally, nothing compares to the liberal, easy-going, entertaining style of the Lonely Planet writers. One of my favorite features of this book are the unique sidebars with interesting facts, trivia or food for thought - like "Top Five Ways to Ruin Your Day at Dusk" or "Did St. Brendan really discover America" and "Shameless Sheilas or Symbolic Shamanesses?". This guidebook is comprehensive, informative and entertaining. When I want to look something up, this is the first guidebook I reach for.
NUMBER 2
Steves is the only guidebook writer that updates his books every year, so the prices and information are the most accurate that can be found. I carry this book along with me in Ireland since the history of the sites I visit is important to me, and Rick Steves (who holds a degree in European History) is a master at briefly framing a site with a historical perspective. Rick writes his guidebooks himself, rather than have a team of writers compile information, and he spent many years as a travel guide. His guidebooks give you the sense that he's is right there with you, leading the way. He'll often direct the reader step-by-step - "turn your back on St. Patrick's Cross, and walk about 100 feet slightly uphill along the gravel path beside the cathedral... you will find yourself at the entrance of ..."
NUMBER 3

The Traveller's Guide to Sacred Ireland: A Guide to the Sacred Places of Ireland, Her Legends, Folklore and People
By Cary Meehan
Sacred Ireland focuses on the hidden sites frequently missed in Ireland that are also the sites that give Ireland its mystical charism, which is the very charm that attracts the traveler. Stone circles, passage tombs, ancient carved stones, dolmens, holy wells, enchanted lakes, ancient ring forts and archaeological ruins are what this book features. Another benefit is the 18 page inclusion about the origins of the sacred landscape of Ireland. This chapter puts history and myth in perspective, and helps the traveler understand the Irish historical timeline. The author is a bit of a mystic herself, and her insights and interpretations make this guidebook a great read for anyone interested in the sacred sites or ancient history of Ireland. This book is a MUST for all those interested in Thin Places.
HONORABLE MENTION

Bord Failte Ireland Guide, 4th Edition
Put out by the Irish Tourist Board, this guide is written from the Irish perspective, welcoming the visitor to Ireland. Rather than giving specifics on prices and listings, this is an in-depth view of the sites and attractions that are constant in Ireland, complimented with endless color photographs. There's background information on Irish music, ancient Ireland, horse racing, the Irish language, Irish flora and fauna, and Irish sports. All 32 counties are included. The book is divided regionally with color coded tabs making it easy to use as a reference. It also details some of the lesser traveled areas such as Laois, Westmeath, Cavan and Moneghan. It is chocked full of photos and insider information for understanding the sites and attractions in Ireland as well as the Irish culture, lifestyle, environment, economy and heritage.