Great recommendation, MickiTee.

Trinity is an excellent fictional account of the background leading up to the Easter Rising. I read it 40 years ago when it was first published, and much of it still sticks in my memory; it was a riveting story. It doesn't seem to be available as an ebook, unfortunately.
For non-fiction, there is a fairly recent account, Easter 1916: The Irish Rebellion by Charles Townshend, which is available for Kindle at Amazon Australia.
http://www.amazon.com.au/Easter-1916.../dp/B002RI9FR2