Previously I had written about Railway retiring room availability: a widespread corruption; but that was for offline bookings. IRCTCtourism.com portal has the feature of booking railway retiring rooms at 483 stations. To avail this feature, first go to the portal link above, or check the list of stations; if your desired destination station is in the list, all you'll need is a confirmed ticket and it's PNR number. Input that PNR number in that page and your journey details will open up automatically.
How to book a railway retiring room online? IRCTC has a page which describes the entire process, have a look at it first. If you care to read the terms and conditions, here it is.
Remember, wait list ticket holders won't be able to book retiring rooms, it is only for confirmed ticket holders and RAC ticket holders. Apart from retiring rooms, dormitory beds can also be booked online.
The rooms or dormitories are provided for a period of 48 hours against each PNR number. The required about for booking can be paid online via various online payment methods and print out can be obtained as your confirmed retiring room booking.