The word is ultimately derived from Sanskrit l/ri/I which means to set free, release, detach, bestow, shatter, melt, drop etc. From this reNu means a small piece, grain of dust, powder etc., but especially pollen. It was used as the name of a number of different drugs, but the name in modern India most likely comes from its association with flowers, a common theme for women's names.
The details of the pronounciation depends on the exact language you are talking of. In Sanskrit, which is almost never spoken as a mother tongue anymore, the stress was on the last short u, and the N was a retroflex voiced nasal (the same relation to the hard d, say, as n is to the soft t). The e was unstressed long, and the r was almost as in English.
In many modern Indian languages, stress has disappeared as a phenomenon. In some, like in Bengali, the difference between the n's is hardly heard any more and the e has become short.