Unable to install locally developed logstash plugin

I have developed a logstash output plugin specifically for our need. This plugin has dependency on cassandra-driver & ione gems. The intent is to install this plugin locally and our servers are hosted on a private network not having access to internet. With this my attempt to install the plugin always fails with the following error message

/opt/logstash/bin/plugin install --no-verify --local <path to plugin gem file>
Installing logstash-output-cassandra
Plugin version conflict, aborting
ERROR: Installation Aborted, message: Bundler could not find compatible versions for gem "cassandra-driver":
  In Gemfile:
    logstash-output-cassandra (= 0.0.1) java depends on
      cassandra-driver (>= 2.1.5, ~> 2.1) java
Could not find gem 'cassandra-driver (>= 2.1.5, ~> 2.1) java', which is required by gem 'logstash-output-cassandra (= 0.0.1) java', in any of the sources.

While I understand from the error that it has missing dependencies, I tried to download the gem file for cassandra-driver & ione from rubygems repository and tried manually installing them as shown below

gem install /opt/sonus/datanode/gems/cassandra-driver-2.1.5.gem
gem install /opt/sonus/datanode/gems/ione-1.2.3.gem

That got me installed the dependencies but after which I try to install the plugin itself still gives me the same error as before. I think logstash maintains its own repository for gems lookup which is located under the path - "/opt/logstash/vendor/bundle/jruby/1.9/gems/". How can I get the dependencies and the plugin installed locally without having to connect to internet. Please somebody help me get through this issue.

Note: By bypassing the proxy I have always been able to install the plugin successfully but I want to install the plugin without having to connect to internet.