I followed the instructions on the Ionic 2 Docs page for installing the Android Studio on my MacBook Pro running El Capitan. Building the android apk went smoothly enough, but the android emulator would not load with the default settings when using the ionic emulate android command.
I finally got the emulator to load by tweaking the Nexus5 (default device) settings to the following:
Target: Android 6.0 - API Level 3
CPU/ABI: Google APIs Intel Atom (x86)
VM Heap: 512
Yes, this is a solution, not an open issue. Hope it saves someone else some time.