how long to stay in Dublin

I went on the 14 Day Ireland tour and spent 4 days before the tour and one day after the tour to go to Newgrange. You don't say how long you will be in Ireland. If you can spare the 4 days that is great and take into consideration jet lag and checking into the hotel. While Dublin is a very nice city and worth seeing, the rest of Ireland is very much worth seeing and spending less time in Dublin is actually more important if you have not much time in Ireland. Seeing the rest of Ireland is more important, fun and worthwhile! I enjoyed Dublin and it is a charming city and plenty to see and do during the the 4 days. But if time is a constraint, then choose more time in Ireland. Layering is very important, and while I went in the end of June, you will need to layer for the constant change in weather throughout Ireland. If Dublin is the only place you are visiting and then going to other places outside of Ireland, then yes 4-5 days is a perfect amount of time. You can go to the jail and to Powerscourt and to Newgrange. There are several good museum and the tour of Trinity College and of course plenty of fun pubs to spend time in. And a play is a good way to spend the evening. Plenty of plays to see and well worth it! Have a great time.

Four days is plenty, particularly if you are going to use one day for a day trip. Newgrange and Knowth are at the same place, by the way. You will take a shuttle bus from the visitor center that serves both to the actual sites. You might also want to consider a day trip to Glendalough, to the south, where you would get a look at the Wicklow Mountains. You will find plenty to do in Dublin for your remaining time. I have been there in shoulder season (April) and in the winter (February). Weather has never been a problem. You are more likely to encounter what they call "soft weather" - misty/drizzly - than outright rain.

Thank you all. I would plan for the trip to be 8-9 days in Ireland total and would definitely include Glendalough.