While all of that may be true, the resources require to take Ireland would be staggering. Looking at the recent screenshots, I count at least 7 units in New Horizons proper, with at least 1 military unit (4 units total) to the south east. The top unit in both stacks are longbowmen. Also, I am guessing that they would have a high rate of production, what with their 10 population, many hills, and iron resource. Our current army is already lack luster, and will be spread out across German (and hopefully Roman) lands at the end of the war. We would have to build from the base up, in order to get a large enough force to take New Horizons, which I believe is on a hill. Our forces would do much better preparing for the worst case scenario of a Russian/Norse/Russo-Norse invasion.
Edit: Not to mention that if they have slavery, they can whip a couple of longbowman, making our lives even more miserable.