Stephen asked above whether you were flying back home from Dublin. One strategy that helps if you're trying to cover maximum ground in minimum time is to fly "open jaw," into one city and out of another, so you avoid backtracking in order to return to where you started. If your visit is going to be to just one relatively compact location (e.g., Ireland), then coming and going from the same airport wouldn't be as much of a factor. If you're thinking of covering more countries and/or a bigger area on this trip in order to see a little bit of everything in Europe, maybe you were already planning on departing from somewhere other than Dublin, but if not, starting in Dublin and ending somewhere else would at least let you cover more ground in just 9 days. Maybe your 1st DD trip involved random stops without a pre-planned itinerary and maybe it worked great, but we've found that minimizing 1-night stopovers, staying at most places for at least 2 nights (even if taking a day trip somewhere else but returning to the same bed) is easier than constantly packing and unpacking.
If you stay in Ireland, are you thinking of driving around the country, renting a car with the steering wheel on the right side, and driving on the left? Are you thinking of getting around by public transportation, bus/train/subway/more planes after you land in Europe? Some methods are faster, some are cheaper, and some can require more advance planning.
We were in Italy (Rome & Sicily) in 2012 for 2 weeks leading up to Christmas. It got dark early, a few sights had shorter hours than they do in the summer, and it rained some days, but it was just cool, not cold (highs in the upper 40's to mid 50's Farenheit each day). In summer, the farther south you go in Europe, the hotter it tends to get. In December, the farther north you are, the colder it could be.