Plesk comes with a way to run Ruby on Rails applications via FastCGI, but it's quite a bit of manual work to setup Ruby on Rails applications and there are currently better, faster and easier ways to run Ruby on Rails applications:
Phusion Passenger: http://www.modrails.com/
Ruby Enterprise Edition: http://www.rubyenterpriseedition.com/ (optional, 33% less memory usage on average)
Phusion Passenger: http://www.modrails.com/
Ruby Enterprise Edition: http://www.rubyenterpriseedition.com/ (optional, 33% less memory usage on average)