St. Nician's Church, in Dublin is certainly not a MAJOR tomb, but very, very fascinating. There is a Crusader buried there, whose mummified remains you can actually touch, as rubbing his finger brings good luck (indeed, I rubbed it and was contacted via email that day that I had gotten a job and was no longer unemployed).

Wolfe Tone's deathmask is on display, as well as the mummified of several other formerly prominent Dublin families.

Finally, it's worth noting that Bram Stoker's family is buried here, and it is likely that his concept for Dracula found its genesis in the tomb of St. Nician's.