Getting a guidebook to Ireland is easy - any large bookstore will stock a few and there are dozens more available to order. From the mundane and basic to the definitely esoteric. But which one is the best? There can be no definite "best guide book" ever. This judgment depends very much on the individual tastes and interests of the user. A guide that is the best for one may not be the best for you.
Ask yourself some questions instead - the answers will steer you towards the best guidebook for you.
Is Your Interest in Ireland Broad or Narrow?
Virtually any guidebook will give you a comprehensive overview of Ireland and point you towards the better-known sights. There is next to no competition here - though some books tend to be written with the social and political history more in focus. The merits of this are sometimes debatable, again depending very much upon the reader's own interest.
Do You Wish to Pursue Special Interests?
Should you wish to concentrate on special interests during your stay you will have to choose the general guidebook most suitable or opt for a specialist publication. Quite a few visitors are taken in with Ireland's mythical past and the early Christian period. Special guides for these areas are available. Their strength is more than often also their disadvantage - by concentrating on their subject the authors omit general and practical information.
Most of the more specialized guides, like those detailing walking tours of Ireland, will need to be supplemented by a general guide for everyday use. Unless you are planning to really focus on your interests only.
Do You Need an Overview for a Short Trip
Size matters - and the bigger a guide the more information is usually contained.
But you should ask yourself if this extra information is what you really need. Or if it would be not only surplus to requirements but also confusing. While I personally think that there never can be too much information in general, the way this information can be accessed can be a problem. If you need the basic facts on e.g. Kilkenny you usually need them in a concise and condensed form. Some books designed for backpackers will give you these plus pages of additional information on hostels, restaurants, nightspots and other details.
Do You Need an In-depth Guide?
If you are planning a short tour of the highlights or are booked on a package tour a short, general guide should be enough for you. You can always supplement this with literature bought locally. If you are planning an individual tour of general interest and need all the information you can get - then the more detailed guides are helpful. They are indispensable for individual travelers who plan on a whim and have no particular agenda when visiting Ireland. In this case, their advantage is obvious. You have all the information you will need at your fingertips and in one handy (albeit sometimes hefty) volume, like the Lonely Planet guide available at Amazon.
Do You Plan to Visit Only a Specific Region?
Should you plan to restrict your visit to a specific region a regional guidebook should be your first choice. Usually providing more in-depth information on the area covered and sporting better maps, these can be much more satisfying than national guides. This is especially true for walking and mountaineering guides, available for several long-distance walks and mountain ranges in Ireland. Or, for Dublin, this "Top 10" guide available from Amazon.
Are You a Very Visual Person?
If the answer is "No" any guidebook will do. If the answer is "Yes" you should consider the excellent visual guidebooks available. They will show you "what others only tell you" (to quote one publisher). This comes at a price. First of all there will be less information per page as the visual elements take lots of space.
This can be compensated by the choice of typeface and size of letters - which might just make them harder to read. A second disadvantage can be the actual price. Visual guides are usually printed on glossy, high-quality paper with four colors used throughout. My personal favorite is the Dorling-Kindersley guide for all Ireland (see it at Amazon).
Can You Actually Use the Book?
Ultimately you will have to look at the actual guides "live". While you can trust all publishers to include the same basic information, the style of presentation has to appeal to you. Check the travel section at your local library or bookstore and see which guide you like. Then read a bit, if possible in low light as well. Take the one you instinctively like and can still read in the shade. And make sure that the guide was published or at least revised less than two years ago, better in the year you are traveling - even the best guides carry old and incorrect information for some time.
Do You Need Detailed Maps?
One purchase to consider when staying in any area for a longer time is a detailed map from Ordnance Survey (Ireland). These maps are not exactly cheap if you need several (due to their layout on a grid system). But they will show you the area of choice in detail down to individual houses, small rivulets, and abandoned buildings in the middle of nowhere. OSi has started to release maps freed from the strict grid system to cater for tourists and walkers in the most popular areas.