The Rails recipes from the Pragmatic Programmers by Chad Flower, has some tips on connecting to legacy databases. That should point you in the right direction, or at least give you some idea of what you're up against
You might also want to look at the ETL (Extract, Transform, Load) approach found in active_warehouse. If you can reduce your dependency to cyclical connectivity (say daily or hourly), you can simplify the process quite a bit. Or, if you could ETL the legacy data and just maintain connectivity for the sake of a few tables, you've reduced the runtime load and complexity of your system quite a bit.