Found this on the Iron.io Github page, which worked for me:

If you're using laravel and see "Class IronMQ not found" error set iron_mq version to 1.* and install dependencies

1.* - Laravel 4.0/4.1/4.2/5.0 compatible, PHP 5.2 compatible version. No namespaces. Using default IronMQ servers. 2.* - Laravel 5.1/5.2 compatible, PSR-4 compatible version. With namespaces. Using default IronMQ servers. If you're not using laravel 4.0-5.0 stick with it. 3.* - Special version for IronMQ API v3. Different API, different server endpoints. Go with it if you're using IronMQ on-premise master branch - same as 2.*