Hi Natalie, I agree with Michael ... Dublin is worth an overnight stay if you can manage it.
I have taken a lot of day trips from London for business leaving early in the morning and returning in the evening. I've never taken a day trip by plane from London for pleasure, because I prefer a more leisurely pace, but everyone has their own preferences when travelling. Make sure that you factor in the travel time and cost of getting to and from the airport as well as waiting at the airport. Keep in mind the possibility that your flight might be delayed cutting into your time. If it still seems worth it to you, then go for it.
As far as must-see sights in Dublin, I really enjoyed the walking tour that is done by History students at Trinity College.