In January I booked flights to Boston flying home from New York. Flight to Boston was €118 and flight back from New York was €153. Total cost of €352 after taxes - would obviously be slightly more if you're flying both to and from New York.
Anyway, booked online with Aer Lingus and travelling in November 2005.
Would also be interested in hearing about good hotels - so far I'm getting quotes of about $150 per night - is that good?
They'll be most expensive mid June-early September and a few weeks either side of xmas... this would be a general rule but you never know there may be exceptions and you might get lucky!
I'd say that October/November is probably a better time from a weather point of view anyway - less likely to have bad winter weather which unsually seems more likely after christmas and right up to March (they had a lot of snow in NY this week)
For the retail side of things I'm sure there are others more expert than me on the timings for sales etc.
For hotels - Cahir I would have thought that $150 a night isn't too bad for Manhattan ... you're unlikely to find anywhere halfways decent for much less.. assuming it's not a prepay rate why not just reserve a room and you can always cancel w/o penalty if you find better nearer the time... November will be fairly low season unless you clash with the Marathon weekend (which I think is in November usually) so there might be good deals nearer the time.