The 10£ fare is probably a teaser, but it appears that Ryanair is very serious about finding a way to acquire long-haul planes and enter the U.S. market, flying to 14 cities within five years, according to the Guardian (UK). They plan to serve the market with flights from Dublin, London Stansted and Berlin.
While many carriers have tried out the low-cost Atlantic market and lost, Norwegian Air is making a serious attempt at it now, and Ryanair is certainly an experienced competitor. Norwegian is currently waiting for an operating certificate for the U.S. for its Ireland-based EU subsidiary; that might be less of an issue for Ryan, whose actual home is in Ireland.
Public domain photo from Wikimedia