It depends on how you plan to use this plugin and the main question: is there a case, where your UI dependencies will not be available, i.e. whether there is a bundle configuration without SWT and RCP bundles?
No UI available
In this case, you'll need to extract the headless part of your plugin into new plugin, which then registers the headless entry point to it. The UI part of the plugin will depend on the new plugin and just delegate UI requests to the appropriate API in the headless part.
In order to provide headless application, you should take a look at org.eclipse.equinox.app.IApplication
interface and respectively org.eclipse.equinox.applications
extension point. When you've defined the application, you launch it by simply invoking:
eclipse -application <app-id> <app-param>
More information can be found in Eclipse Help.
UI available
The simpler case. Only the headless entry point needs to be specified and everything will work as previously.
My experience, however, shows that sooner or later, the case arise where the plugin needs to be split and depending on its complexity it might cause more trouble than it would have been if it was split earlier.