If you are just trying to use a different robot with MoveIt!, you don't need a new IK plugin. The IK plugin is what Moveit uses for computing the inverse kinematics and the default one uses KDL which is a generic solver that should work for any kind of robot arm.
All Moveit! needs to work with a new robot is your robot's URDF file and you have to go through the MoveIt! setup wizard.
If you already knew this and have tried the default plugin and are not happy with the performance, there are two easy alternative IK plugins that I am aware of, the OpenRave IKFast plugin and the Trac-IK one. The IKFast method creates a solver that is custom to your robot since it is an analytical solver, Trac-IK is similar to the KDL one (both are numerical solvers) which should work for all robots.
If you have went through the trouble of solving the analytical IK solution for your arm by hand, you can make you own plugin from scratch, but that is not too easy.
Long story short: You shouldn't need to make your own plugin unless you've tried the default one and are not happy with it.