A long fetch creates a high energy wave
The fetch, also called the fetch length, is the length of water over which a given wind has blown. Fetch is used in geography and meteorology and its effects are usually associated with sea state and when it reaches shore it is the main factor that creates storm surge which leads to coastal erosion and flooding. It also plays a large part in longshore drift as well.
Fetch length along with the wind speed (wind strength) determines the size (sea state) of waves produced. The wind direction is considered constant. The longer the fetch and the faster the wind speed, the more wind energy is imparted to the water surface and the larger the resulting sea state will be.
United States to Ireland 
The winds which travel from the East Coast of the United States and hit the west coast of Ireland would have an extremely large fetch and would in turn produce very large waves if the wind speed was high and wind direction constant over the duration of the fetch.
Cup vs. Bowl of water 
If you blow across a cup of water you will notice small ripples form on the surface and hit the far edge. The fetch distance is the diameter of the cup and wind speed is how strong you are blowing. If you blow at the same strength over a much larger bowl of water the ripples will become larger when they hit the far side of the bowl as there is more distance and thus more time for the blowing to impart energy onto the water to create the larger ripples.
See also