Strictly speaking, it is not stated categorically that Duinhir was descended from the Dúnedain. However, his Elvish name and his notable height do seem to imply that this was the case.

All we're told about Duinhir's seat is that he came from the uplands at the head of the Morthond Vale (that is, the more mountainous lands at the northern end of the valley). This would place his home somewhere near Erech, but still at some distance from the Stone of Erech (we're specifically told that none of the valley's people dwelt close to the Stone).

'River lord' (duin hîr) is not a canonical explanation for Duinhir's name, but it does seem to fit the name's structure. It also makes considerable sense in context, given Duinhir's position as lord of the valley of the one of Gondor's more important rivers.