Get a British Admiralty or a U.S. Pilot. I am guessing that mid-June is the best time (most steady wind with fewest gales) on average, but pilots only report the average historically: one year with four June hurricanes turning into six gale front at 45 N cancels out another year with a single June 30 30 knot gale off Iceland and dappled calms the rest of the month.
A catamaran, especially a 50 footer, is a fast boat. If the idea is to avoid heavy stuff, but to keep moving, I would suggest the shortest circle route, maybe in July to avoid bergy bits out of the Davis Strait/Labrador. "America" here could be the entire East Coast, but a lot of people would go to St. John's, Newfoundland and, knowing they could maintain 10 knots, would pick a likely weather window and get to Ireland in 180 hours or eight days or so...eight days being on the outer edge of a semi-reliable forecast.
We monohullers just have to leave when ready and hope we don't get plastered more than once in a 15-20 day crossing...