During a recent trip to Ireland, I had a chance to visit and tour St. Michan's Church in Dublin. Built in the year 1095 AD, it is one of the oldest churches in Dublin. It is also associated with much of Ireland's history from Handel who practiced the "Messiah" on the organ to Bram Stoker who is believed to have visited the vaults with his family. The tour also includes the famous vaults below the church. The crypt contains the mummified remains of some of Ireland's past. These include the "Crusader" and the legendary Shears brothers.
Tours are available throughout the year although times will vary. Check ahead of time to see if tours are available the day you wish to take the tour. Keep in mind this is still a very active church. The vaults are not wheelchair accessible and photographs are not permitted in the vaults. There is a small fee but it is worth every euro! The staff is very friendly and willing to answer questions.
You can find out more from their website: http://www.cccgroup.dublin.anglican.org/Christ_Church_Cathedral_Group/St._Michans.html.