Ubuntu – Ubuntu installed GRUB over rEFIt

Did you reinstall rEFIt manually?

If you didn't, try running this on the OSX Terminal

cd /efi/refit   
./enable.sh

When prompted for your password, use the one Yosemite asks you for while trying to install new software.

After that, you should see the boot menu after you restart. When you choose to load Ubuntu from the boot menu, you may be greeted by the grub rescue> terminal" instead of a normal boot, let me know.

By the way, you can also press the option key (alt) just after you hit the power button, and there you'll see some options, including rEFIt. This has the drawback that it must be done on every boot, hence the preference for the terminal solution.

Both your #1 and #2 options are possible; however, if you don't understand the Mac's native EFI-mode booting and how the Mac implements BIOS/legacy booting, you're likely to make a hash of things in setting it up. My suspicion is that you installed Ubuntu in BIOS/legacy mode with a BIOS-mode version of GRUB, which means that GRUB will be unable to launch OS X (or maybe it'll manage, but that path is very flaky in my experience). If I'm right, you'll want to install either an EFI version of GRUB or rEFInd to do as you want. Installing from OS X is preferable, although the Mactel tools to which LiveWireBT can theoretically make this more reliable in Linux. (I have limited experience with these tools myself, though, so I can't comment on them in detail.) My pages on hybrid MBRs and EFI boot loaders for Linux will help bring you up to speed, but be prepared to spend some time on the reading. Sorry, but I know of no good shortcut for this. A brains-off step-by-step guide might work, but is more likely to lead you astray because of assumptions the author makes that don't apply to your system.

The rEFInd documentation is pretty extensive. Be aware there have been significant changes in rEFInd's Mac support recently. In particular, version 0.8.4 and later now install to the EFI System Partition (ESP) by default, which is necessary to work with the way Yosemite is set up by default. This change has caused some confusion among users, though.

Full disclosure: I maintain rEFInd, so I'm not unbiased.