How great that you and your wife are visiting Ireland together. You already know that winter is not ideal and days are short but that will give you more cozy pub time. Plus you can be romantic and cuddle up in front of a roaring turf fire in many different places.
You could take trains from Killarney to Cork to Dublin. Take a look at the various schedules and prices to see if it is worthwhile for you.
Personally, I would pull a day from Cork and add it to Killarney. That would give you 3 in Killarney and 2 in Cork. Let's see what the rest of the forum members think of that idea.
Michele,
Thanks for your tips (as always). I know the days are short in winter but this time or we take this chance or miss it at all. So we prefer going to Ireland in winter !!! I will think about staying in Killarney a little longer as you suggested.