I like to carry my journal with me wherever I go so I can document my experiences. I also like to take along a good guide book, like the DK Travel Guides. Prior to any trip I like to do some preliminary research on the locations I would like to see, opening times, fees, transportation to get me there, closeness to hotel.
I like to also have a rough itinerary especially on a short trip.
Miscellaneous: The Rough Guide to Dublin is pretty cheap and has a lot of detail about attractions, pubs, restaurants etc. The maps in the back are excellent and east to use. I found out so many things that I would never have noticed just walking around. Definately worth the money.
Miscellaneous: The book "Round Ireland with a fridge" written by Tony Hawks is an useful companion for an Ireland trip. It tells the true story of a bet that Tony cannot hitch hike around the circumference of Ireland with a fridge within one calender month. On his trip Tony visits many irish places and their unique people. The book is easy to read and gets you laughing out loud.
The book "McCarthy's Bar" written by Pete McCarthy is a fine travel book for an Ireland trip. Pete McCarthy was born Irish but has spent most of his life in England. He therefore decides to do a tour from Cork to Donegal. On that trip he discovers that despite many changes since his childhood, the charm of the Irish people has still remained.
The book is not a tourist guide but it gives you a fair view of Ireland and its people.