I read an article from a recent study of remains that were discovered to be most certainly Xiongnu.
http://www.nature.com/jhg/journal/v5...jhg20108a.html
The DNA testing of these remains has identified them to be strongly tied to modern northern Asians, indicating a Siberian lineage. Other reading on the subject has had one scholar actually suggest that the Xiongnu language may be related to Yenesian - a Uralic Siberian language. Also, an article here has identified a male in a Xiongnu grave site to be Indo-European, specifically Saka-Scythian. There are indeed evidence that a Saka horse-archer culture invaded the Ordos culture.
This is leading me to lean in favor of the theory that the Xiongnu were multi-ethnic and that a large amount of the remains tested seems to suggest Siberian origin. Others among the Xiongnu we now know were Iranic Scythian, and the Turks who first appeared from the Altay Mountains on the Western extremity of the Xiongnu realm. That these nomadic peoples migrated southwards and went so far as to be part of the same peoples who plagued the Han Chinese with raids for centuries.
On the other hand, modern Mongolians seem more or less tied to the Donghu, who were tied to the region of Manchuria and were later themselves destroyed the Xiongnu. (Even the title 'Khan' originates from the Donghu of Manchuria) It gives me doubt that the original Xiongnu were Mongolian in the sense that we think of it today. Of course, I am an amateur on this subject and I may be entirely wrong. It is likely also that this so-called Ordos culture merely mixed with Xiongnu people and eventually became the dominate people who are the Mongolians today.
It seems, to me anyway, that the Xiongnu are most especially a mix of Siberian, Turkic, and Saka.
The history surrounding the Xiongnu (and by extension, the highly disputed pre-history of Huns) seems a lot more complex than originally thought. I have doubt that the Xiongnu really were tied to the early history of the Huns, but rather that in actuality many groups who were subjugated by the Xiongnu later became the Huns.