Can Dash from Fate Reforged be used at instant speed?
The official rules are not out yet, but my guess is no. The templating on the reminder text strongly suggests that "Dash" will function as an alternative cost that does not affect when you can cast a spell. In this regard, it would be similar to the "Evoke" mechanic.

For creatures with flash, you would be able to "Dash" at instant-speed (similar to Briarhorn)