In the words of the website:
2011 will be the sixth year of the Dublin: One City, One Book project, designed to encourage everyone in the city to read the same book during the month of April each year. The project promotes reading in a city which boasts one of the world’s greatest literary heritages including four Nobel Laureates and was a major element of Dublin's success in becoming a UNESCO City of Literature.
This year's book is Joseph O'Connor's highly-acclaimed Ghost Light, the story of the love affair between playwright JM Synge and the actress Molly Allgood. Last year's Irish Times review is here, although personally I'm going to read the book before I read any commentary or reviews.
(I'm going to pick up a copy from the Gutter Bookshop in Temple Bar.)