Just returned on Friday, from a two-week trip (11/1 - 11/15). We traveled most of the country. Always carry an umbrella - weather will change every 15-minutes! The coasts are very windy (we experienced a day of 40-60 mph gusts). In general, I found the 45-50 degree weather to be more comfortable than I expected.
Wear layers - I primarily wore either: a) a fleece jacket with a light long-sleeve wind blast jacket, b) sleeveless wind vest under a light long sleeve wind blast jacket, or c) just the light wind blast jacket. I also brought a down sleeveless vest, which I found too warm and did not wear. (PS. The fleece makes a great pillow on the plane.)
I purchased a tuque (winter pull-over hat) to play golf. The wind is so strong on the coasts, that it is difficult to keep your hat on!
As indicated by a previous post, I think October may be the best month of the year, but we loved spending time with the locals. The roads were not heavily traveled, and we had no trouble with lines at the attractions (we had advance tickets, but we could have just walked in Guinness, which is unheard of in the summer months).
Hope this helps! You will have a blast.
Ron