Since you brought this up khushi, i have to tell you folks.
I think Kirat denotes to all the Nepalese and not just rai, limbu, etc who have been identified by scholars long time ago.
Kirat Pradesh has been found in many earlier documents before Nepal. Hence i am sure that Kirat or Kirati means to all the Nepali people living in Nepal now or have been.
The bishnu purano, one of the ancient text also mentioned that caste system was absent in Kirat Desh(Nepal). Hence regarding this i am confident that the existence of several ethnicities was due to this system which arrived in Nepal later.

I read in one of book by Dor Bahadur Bista on how the ruling class wanted to divide people/group into caste system.
I also read "The History of the Kirat People", 2003. It throws severals light on how several ethnicities came to be in Nepal. For example, the book states that Kirat people came to Indian sub-continent and China from Assyria in Mesopotamia.
The first group that entered the Indo-Gangetic Plain and the Himalays regions was the Kiratas.
The second group and third group that migrated from China/Tibet and Mongolia into Nepal respectively are also mentioned.
The assimilation of second and third group of people with the Kiratas resulted in several ethnicities.

But it can be seen that all these 3 groups are the same and have been living in Kirat Desh viz Nepal.