First... somebody suggested the buckle is Confederate -- but, of course, no Confederate buckle had the Union's motto ("E Pluribus Unum") on it. The buckle's owner said above that it has E Pluribus Unum on its emblem. (My point is, please keep in mind for future reference, anytime a relic has E Pluribus Unum on it, it is definitely NOT a Confederate-made relic.)
Creskol has already posted most of the relevant historical information about this buckle, but its owner, Kuger, sent me a Private Message asking me to comment about it. So, I spent about an hour researching it, with the following result to add to what Creskol said.
1- It is a state or local Militia buckle, not US Army issue, because on a 2-piece (meaning, "tongue-&-wreath" type) buckle the presence of stars around the eagle is contrary to US Army Regulations of that era.
2- According to the book "American Military Belt Plates" (pages 92-94), the presence of the Union motto "E Pluribus Unum" with the stars moved to below the eagle date it the mid-to-late 1830s into the 1840s.