Dublin City Library and Archive has produced a useful list of its 1916 Rising resources.
The repository's material incudes eye-witness statements, newspaper collections, electoral registers 1898–1916 (1908–1915 are online), the Monica Roberts Collection (also online), the archives of the Royal Dublin Fusiliers, and the archives of Jacob's Biscuit Factory. The latter is due to be made available online this year.
The Monica Roberts Collection comprises some 453 letters from 56 men serving in the Royal Dublin Fusiliers and the Royal Flying Corps. They were sent in response to letters from Monica, a young woman from Stillorgan, Co Dublin, who sent letters with comforts such as tobacco, chocolate, socks etc to Irish men at the Front. Many of the recipients replied to her, giving a vivid picture of conditions at the Front and commenting on contemporary politics including the 1916 Rising. Each item has been digitised and transcribed and can be searched and viewed online at the above link.
See the DCL&A's 1916 Resources here.