This is a heavily Christainised site. The Fire Temple is where St. Bridgit's nuns tended an eternal flame that was put out in 1530 by Norman invaders.
This tradition of the eternal flame must have dated to far older than the early Christain period and its association with St. Bridgit/ the Goddess Brigid make this an important site. There is a vault called "Bridgits Kitchen" as well.
The round-tower costs €5 to go into and is one of only two round-towers in Ireland that has been restored for use as a tower. There is a charge to enter the catheral as well (I can't remember how much it is). Bizarrely the tomb of one of the bishops of Kildare from the 16th Century has a Sile-na-Gig on it!
Visiting times are from May-Sept (definitely May not sure about Sept) from 10-1am and 2-5pm. Kildare can be got to easily from the M7.
I know there is a lot of folklore attached to this site, the book "How the Irish saved Civilisation" has a good section on the historical Bridgit. I've loaned it to a friend, but once I get it back I'll update this section.