Let me follow BW's example and focus a bit on the Sumerian King Lists. (Much of this from Chapter 16 of "Historical Genesis From Adam to Abraham" by Richard Fischer)
In his book, Fischer does a comparison of multiple lists of the pre-Flood Sumerian kings and reconciles them into a single list of ten pre-Flood kings.
1. Alulim
2. Alalgar
3. Enmenluanna
4. Enmengalanna
5. Dumuzi
6. Ensipazianna
7. Enmenduranna
8. Ubartutu
9. Su-Kur-Lam
10. Ziusudra
Scholars have been interested in the possible relationship between the Sumerian kings and the Genesis 5 patriarchs because the Sumerian Kings and Genesis 5 Patriarchs:
- all had extraordinarily long life spans
- lived in Mesopotamia
- lived during the same pre-Flood Period of time
Richard Fischer does not believe that all 10 of the Sumerian pre-Flood kings are the same people as the 10 Patriarchs listed in Genesis 5. But he does believe that there is a possible link between the last 4 pre-Flood Sumerian kings and the last 4 Genesis 5 Patriarchs.
He believes that it is possible or likely that...
Enmenduranna, Ubartutu, Su-Kur-Lam, and Ziusudra from the Sumerian King lists
are
Enoch, Methuselah, Lamech, and Noah from Genesis 5.
So in answer to the OP, I believe that some of the pre-Flood Sumerian kings do have some potential relationship to some of the Genesis 5 Patriarchs. And the Sumerian King lists potentially provide some corroboration that prior to the Flood there were some people living in Mesopotamia who had extraordinarily long life spans.
So there does appear to be some relationship between the pre-Flood Sumerian kings and the Genesis 5 Patriarchs.
Now to the Nephilim...
Are the Nephilim and the pre-Flood Sumerian kings somehow related?
Let's start with the Scriptural text.
Genesis 6:1-4
6 Now it came about, when men began to multiply on the face of the land, and daughters were born to them, 2 that the sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose. 3 Then the Lord said, “My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years.” 4 The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown.
The text tells us the following about the Nephilim.
- They appear to be the offspring of the "sons of God" and the "daughters of men"
- They were mighty men and men of renown
- They appear to have had a life span of 120 years.
I think we can use the life span of the Nephilim to distinguish them from the extraordinarily long lived Sumerian kings and Genesis 5 Patriarchs.
The Patriarchs listed in Genesis 5 had lifespans ranging from the 700s to 900s.
So the life span of the Nephilim (120 years) is significantly less than the life spans of the descendents of Adam listed in Genesis 5.
On the other hand, the median life span of Neolithic humans was between 30-35 years. Even if we take into account the effect of infant mortality on this number, the 120 year life span of the Nephilim was higher than the median life span (30 - 35 years) of Neolithic humans.
All that to say this...
Since the life span of the Nephilim is significantly shorter than the life spans of the Genesis 5 Patriarchs, I think it is unlikely that the extraordinarily long lived pre-Flood Sumerian Kings are related to the Nephilim.
However, I would say that there is a probable relationship of some sort between the pre-Flood Sumerian Kings and the Genesis 5 Patriarchs.