Basically, it seems like Dallas had several players targeted and traded down to take them in a spot where they would be the BPA.
For example, Ireland (I think it was him) talked about how they were considering trading down from pick 18 but knew that both guys they were considering would probably be gone, so they just took the guy they wanted.
Then, in the second round, they were apparently considering OL, but when the board dried up, they saw that they could trade down and get Fasano in a spot that made sense for his valuation. Same thing in the third with Hatcher.
This is excellent strategy, assuming that the Cowboys were somewhat aware of the valuation that other teams had placed on the players that were targeted. You might disagree with the valuation, but once that is set, this strategy is pretty optimal.
The final thing I would add is that Parcells puts a lot of value on a few characteristics like toughness, discipline, and love for the game that are perhaps not considered (but at least not valued as highly) by other coaches and organizations, and that may just rule certain players out altogehter. Knowing that other teams will take these players regardless of what this organization deems fatal flaws certainly affects the way Dallas is willing to work the boards.