Apple - Is it possible to enter emojis directly using a keyboard shortcut?

Go to System Preferences → Keyboard → Text and set as many replacements as you wish.

Set the ‘Replace’ to the standard characters and the ‘With’ to the emoji you would like the ‘Replace’ to be replaced with.

These synchronise over iCloud, so will be available on your iOS devices too.

There is a nice application called Rocket which enables you the Slack-way of typing emojis:

You type : (colon) followed by the emoji's name and it offers a popover with autocompletions.