Why and when
Anytime really. Most likely it will be raining so why plan ahead. Whatsoever, spring or summer months is your best chance, if you want to escape the cold months.
Getting there and around
In Dublin city, you can just walk around the city centre. If you are interested to visit the countryside consider renting a car for a day.
Pubs and restaurants serve great food. Potato is a popular ingredient for a lot of dishes but tastes better than ever.
Let’s say it is more expensive than anticipated. Dublin has a huge housing problem, thus hotel prices, as well as Airbnb, are equally expensive.
A free walking tour is the best way to get around the city and learn some history. The guys at the tour are very friendly and during the walk you have fun and you get to learn about the older and most recent history of Dublin, which is very interesting.