Internal or not, IOS apps are distributed via Apple iTunes or a locally hosted enterprise digital rights management system. The reasons for this are to control the quality of apps that get installed, as well as to control the flow of money. Yes, I know...it's a free internal app, but controlling the flow of information is worth billions to Apple.
Here's the basic steps:
1. Your company signs up for a volume purchase account (to id the company as a single org)
2. You build and publish the app on iTunes and specify it is a B2B app, and list who can "buy" it.
3. Your company "purchases" the app, which then allows devices associated with the VPA to install it.
More information here:
http://www.apple.com/business/vpp/
Google has a similar method called private channels for android apps.
http://www.techrepublic.com/blog/google-in-the-enterprise/deploy-private-android-apps-on-google-play/
The alternative to these methods is posting the app on a file server somewhere with instructions on how to side-load the app like a developer would do for testing. I don't recommend this because updates would be a pain.
Lastly, if you don't need to run the app offline or need native only features like the camera, you can always look to publish the app on a internal web server instead...and have users "bookmark" the url (which makes it run full screen when launched).