I don't know if anyone is still interested, but I got ruby on my Nokia N810, along with irb, with supreme simplicity I might add.

I was able to install ruby by wget the deb package from https://garage.maemo.org/projects/ruby185

wget gave me some message about SSL on my first try and suggested I try the option that does not check certificates, and that worked just fine.

Once Ruby was installed, I simply scp'd the irb script (Yes, it's just a simple, simple script) into the /usr/bin/ directory. I had to make one modification to the irb script in order to get it to work, but it's easy and is practically a gimme. At the top of the script is the she-bang, literally "#!" followed by a path to the ruby executable. I copied my irb script from a Debian server so it was pointing to /usr/bin/ruby1.8, which wasn't working because ruby on the N810 is /usr/bin/ruby. Guess what you have to change :P

Simply change #!/usr/bin/ruby1.8 or whatever yours says, to /usr/bin/ruby which is where that .deb package seems to install it's script to.

Then, as long as irb is executable, and it should be, then it will work. Happy Ruby-ing!