Welcome to my forum. Most places won't take reservations till after the first of the year. With the recession and economic downturn in Ireland you are probably okay with booking a month or two in advance. However, if there is a festival, horse racing or something important going on in a particular town you should book as soon as you can after the 1st.
For hotels you are interested in, sign up for their newsletters. They will send out special offers that may be of interest.
Definitely book flights 21 days in advance or more. Sign up for newsletters for your airline of choice. Check the website frequently. Clear cookies from your computer in between each new website check. When you find a price you can't resist, jump on it immediately. They don't usually last long.