How to go to Dalaran from Ashran?

Assuming you aren't a mage and can't teleport yourself:

The fastest way to Dalaran is to teleport yourself with one of the sixteen rings of the Kirin Tor. If you played during Wrath, you might still have one laying around in Void Storage. If not, well, they start at 6800 gold (at Exalted with the Kirin Tor). Visit Harold Winston to buy one.

You can also use Jaina's Locket to create a portal to Dalaran on a one hour cooldown. This can sometimes be found for 10K gold or less on the auction house, or you can run the Shadowmourne legendary quest chain and get your own.

The second fastest way is to ask in chat while in Ashran or your garrison and buy a portal to Dalaran from a mage.

If you played in the Argent Tournament, you can use their Argent Crusader's Tabard to teleport to the tournament grounds, and Dalaran is a short flight away.

If you're an engineer, you can use a Wormhole Generator: Northrend to get to Northrend, and then fly from Storm Peaks to Dalaran.

If you're a monk and can fly in Pandaria, you can take a Zen Pilgrimage to the Peak of Serenity, then fly to the Shrine, then take a portal to Dalaran.

Failing the above, my suggestion would be to take the portal to Stormwind/Orgrimmar, then the portal to Pandaria, a flight to the Shrine, and then the portal to Dalaran within. This should be faster than taking a ship to Northrend as you won't have to wait for it, and the flight in Pandaria is shorter than the flight in Northrend.