Do you have a source? I'd like to have a sneek peak of where else they are going
As of right now Islip does not have any international customs facilities, they are purely domestic flights. The 787 will be in the area of 757/767 size.
The 757 requires 2,365m (approx 7759 ft) of runway to takeoff (maximum) , and 1,400m (approx 4593 ft) to land (maximum). The 767 needs 2,790m (approx 9051 ft) to takeoff (max) and 1,680m (approx 5511 ft) to land (max).
In turn, Islip's longest runway is 7006 ft long, so the 787 could no doubt land there, but it would most likely not be able to take off again. Unless Islip Airport lengthens their runway, it simply would not be possible. Also keep in mind that the FAA requires overrun areas at the end of the runway in case the aircraft cannot stop in time. The overrun area prevents the airplane from going onto streets, houses, etc. Also please note that Ryanair may end up going with Airbus and ordering the new A350
Again I would like to see the source of the info about Ryanair flying to Islip, I would think they would fly somewhere a bit closer to NYC but I may be mistaken.
Hope this explains a bit.