There doesn't seem to be any magic time for buying trans-Atlantic tickets, other than not at the last minute. Fall 2016 was a very unusual situation, with multiple cycles of fare dips that offered prices hundreds of dollars below the usual. There were some shockingly low deals not just from the usual gateway cities like NYC but also spotty incredible bargains from many other origins. Demand for at least some European destinations (due to terrorist incidents??) was looking soft, and the airlines cut prices.
There's no way of knowing what will happen as we go forward. If you monitor the fare on the route you're interested in for a little while, you may spot a temporary dip. On the other hand, you may watch that fare drift upward and end up paying more. I agree that a return date so close to Christmas makes things dicey. Since I haven't ever monitored fares from California to Dublin, I have no idea whether what you're currently seeing is a pretty good deal or not. Without any context, it doesn't sound bad at all to me.
I hope someone who travels to Ireland with some regularity will come along and give his or her opinion.