One of my favourite days of the year is Handel's Day, 13th April. Temple Bar Cultural Trust have organised a range of events for the day with the highlight being the wonderful Messiah On The Street, an open-air performance of Handel's Messiah on Fishamble Street. There's always a lovely atmosphere at Messiah On The Street, and it's a genuinely unique event in that Fishamble Street was the site of the original world premiere of Messiah back in 1742.
The performance runs from 1:00 pm to 2:00 pm. It's very popular so I'd suggest getting there early to get a good spot.