iOS iCalendar (ICS) Integration I am using an ICS-based solution for Mozilla Lightning, using FTP to access and modify an ICS file.

How can this be integrated into the iPad?

My URLs are of the form: ftp://username:password@domain.com/calendar.ics

I am open to alternative solutions such as automatic conversion to webCal or something similar. Please note that I need the calendars to be continuously synchronized.
A .ics file is effectively a read-only solution. An iOS device can import a calendar that way ("Subscribing" in Apple's terminology) if it's hosted via HTTP, but it's a static file from the device's point of view. It only gets updated if the file at that URL gets updated, and the iPad can't add/edit/delete items. It's the digital equivalent of the flyer with your garbage collection schedule on it - you can read it, but you can't change it.

To get what you seem to want - a central calendar that one or more clients can access and update, you'll need some sort of calendar server software to keep everything in sync. CalDAV is an open standard that's well supported by the built in iOS calendar and most apps on other platforms.

I don't know what server software is good, but there are several options to choose from. One alternative to running your own server is signing up for iCloud on your iPad and accessing the CalDAV server behind it from your other calendar clients (Lighting, etc.). It looks like this can be done without too much trouble, but I haven't tested it - I found a blog post with instructions here.