Best time to buy?

It's always a guess. I start out by looking at options and tracking prices for awhile first. That gives me a good idea what a decent price is and what potential options are out there (airlines, airports, times of day, duration). If I see something well below the decent price that works, I'll jump on it. If not, I'll watch seat availability on the itineraries I like best but wait to see if the price drops. If I get to two months, 60 days, out and haven't purchased a ticket, I'll book something at the decent price.

The best time of buy is always yesterday.

Where are you flying out of?
If you are anywhere near NYC, check out Norwegian Air out of Stewart Airport. they are cheap enough that you need not buy ahead. .

In addition to asking from where you're flying, I'll add are you looking at a specific airline, or for anyone who can fly you there? Norwegian, which dkgibbons mentions above, has just started serving the Denver airport, and while it doesn't offer a direct flight from Denver to Ireland, it offers a relatively cheap but comfortable way to get there, with a connecting flight. For folks flyng out of Denver, they are giving some serious competition to British Airways, Lufthansa, United, and Icelandair. Each of those can have its own policies regarding ticket flexibility, price fluctuations the longer you wit to book, and refundability. The general rule is, if you're ready to book a flight and you find one at a decent price with the departure and arrival times you want, book it and then look forward to your trip.

The best time to buy any airfare is when you've satisfied it's the best you can get and are happy to pay it.