Dallas is a former member of Reilly's Rangers who died in 2277.
Information about Dallas can be found on Reilly's terminal in the Ranger Compound, as follows:
It's been a month since we lost Dallas and Kira. They were assets to the Rangers and even though I've taken Theo on board to bolster our numbers, he's hardly a replacement.
Dallas is mentioned in Fallout 3, but does not actually appear.