Ah Louise, I was in Portugal sure!
Jake, I just replied to your London one and then saw this!
Cheapest way from London to Dublin is via
skyscanner (this will search all the airlines for you, but you should book in advance as it gets more expensive towards Paddy's Day
Temple Bar is the main tourist drinking area - but it will attract higher prices for that reason, and it will be noisier, depending on what you want. To book a hotel, try and get one with a Dublin postcode that is 4 or less - then you'll be city centre. 'Dublin 1 &3' are north of the river, and 'Dublin 2&4' are south, but all are central.
Late rooms is a good site
It's not the best place for St Paddy's Day (Temple Bar) - you want to be out on the streets for the parades etc, and it will be really obvious where this will happen - just follow the crowds!! In the evening it will be good - but packed!!
Things not to miss - Kilmainham Gaol - for history (but very good!); Viking Splash (for the craic!); the Rock n Roll bus tour!