For a 1916 tour can recommend Lorcan Collins, he's knowledgeable and affable:
www.1916rising.com
If you can, do a tour of Kilmainham gaol. It was closed after the civil war and abandoned for nearly forty years, trees were growing up through the floor. At the start of 60s a group began a voluntary effort to save the building, relying on the labour of many republican tradesmen etc. For decades the same group also ran the tours of the site with an unusually radical flavour. At some point in the 90s, IIRC, it was handed over to the Office of Public Works.
In addition to its intimate connection with the history of separatism, there is also a very interesting museum which is basically an examination of the social causes behind incarceration in Dublin. Lastly, it's a really iconic space which has been used for many music video and film shoots, very evocative.
For some reason it's become incredibly difficult to get tickets, whenever i look it's booked out, so try and familarise yourself with the release dates and get in early.
Get a LEAP card, you can use it on the LUAS, DART and buses.