If I were you I would stay overnight. There is a lot to see in Dublin but it is compact and you could see a fair bit if you had a morning flight and a late flight in the evening.
On my last visit I stayed at the Academy Plaza hotel just off O'Connell street. Best Western but refurbished 2007/2008. It cost me around £75 for the night and so not too expensive. Standard good and you can read many reviews on trip advisor and on perhaps here also. I think I did a review on both sites certainly on Trip Advisor.
Get the Airport shuttle bus into town which is not expensive or if you want to do it cheaper get the normal service bus in. If you are going with say Ryanair you can fly for £1 right now each way and there will also be plenty of offers all this year I feel sure.
An overnight stay would let you check in on line and just take hand baggage and then it would literally just cost £2 each return.
From this hotel the tour bus leaves right across the road and this will take you round the city and let you choose which places to get off and visit. OR you can do it all on foot or use the normal buses. The Euro is very strong right now so not a lot of difference between this and the £.
It is not cheap to eat out in restaurants but you can eat cheaply if need be and also drinks are expensive.
If you as suggested look at the Dublin Tourist web site lots of ideas. Me I did a lot in that I spent 3 days at the Camden Court Hotel nice hotel but a bit further out. If you can push the boat out there are some great 5* hotels but expensive.

Sue