For RHEL 6, you will need to enable EPEL repository, with a command like this:

sudo rpm -Uvh http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm

Then install Ivy (this should work on Fedora too):

sudo yum install apache-ivy

Note: The Ivy client will download required files the first time it is run, which may take several minutes. It will show resolving dependencies... while this is happening.

Other systems

Download an Ivy binary distribution, extract Ivy's jar file somewhere, and set the environment variable IVY_JAR to point to it.

For example: export IVY_JAR=~/apps/apache-ivy-2.3.0/ivy-2.3.0.jar