How can I install and run R on an iPad?
It's actually pretty simple, in that, if it is not in the app store it can't be installed, unless you want to go the jailbreak route because you found an "R" app for jailbroken IOS devices.
There is an app called Analyser by You Hui Yan available on the App Store ($14.95) which includes a R interpreter as well as a Python interpreter. The install.packages() is disabled (per Apple's requirements), but a fairly broad number of libraries are included. The last update was in 2017, so it is still active.