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Try the Genesis Plus GX core in Retroarch. You need a Sega CD BIOS file. For more information look here:

http://wiki.libretro.com/index.php?title=Genesis_Plus_GX

There's also the MD.emu app on the Play Store. It's paid but there is a free version (MD.emu Free) compiled by another dev. Since the emulator is open-source it should be identical to the paid app.

I have the paid version since I appreciate the work the guy does on the emu's, though he isn't the original coder and apparently there are some people that aren't happy that he charges for it, even though it's perfectly within his rights.

Anyway the RetroArch core is probably better.

thanks for the tip! I hate retroArch with a fiery passion, so I wanted to go with md.emu. Problem is, md.emu free has not been updated for over 3 years. Anyhow, I installed another version which is newer (also a free fork), it's extremely clear from the menu and everything it''s just md.emu, but I got this error message when trying to load silpheed.iso: 

unknown cue sheet directive

Whatever does that mean?

I also tried androgens which I normally use for all non sega cd megadrive games such as gunstar heroes and all of those run perfect and are tons of fun on the XD, but it doesn''t run sega cd games apparently...

But what is that error in md.emu  "unknown cue sheet directive" ?

Sega CD used audio tracks in the cd for the music.
You can find different rips of the games, some come with the game data as a .iso file and the music tracks as mp3 files. You need a .cue file which is a plain text file that tells the emulator the exact order and duration of the mp3 files as they were in the original cd. It is a known nuissance with sega cd, just google it.
Or, you can download a .bin rip, that merges both the data and audio in one file, they are an exact copy of the entire cd. They are bigger in size though.

WOnder if anyone can help, I just downloaded Retroarch but the interface is totally all over the place..  I can't seem to navigate anywhere properly, some joypad buttons move forward and another exits but largely wth?!  I'm running Skelton's custom, any advice?

From a high level, get the latest revision from the retroarch website (google it).  Once installed you go to options and to inputs and there is something like "input all" where it will run through every button and you touch the button the GPD that you want to map to that.  Once your keys are mapped you update the cores which are the emulators, then point retroarch to the root of where all your roms are (mine are on /mnt/external_SD/SNES,NES,GB,etc).  I'd also suggest mapping the RGUI button to R3 Left or R3 Right - this allows you to get to the save/load state screen after you start a game.

For me details check out this setup link -- http://lifehacker.com/how-to-set-up-an-all-in-one-retro-game-emulator-with-re-1745863517

If that one doesn't work for you, google "How to setup Retroarch" and you will get a bunch of videos and links.

After I understood the basics I put up above, retroach became easier to understand.  I now use it for emulators I don't want to download like Atari 7800 EMU.

Its good but not quite to the point where I would dump all my individual emulators just yet.  I wish they had the Muppen64 AE+ core for Nintendo 64.  The one included didnt work for me, but stand alone Muppen works just fine.

Hope this helps

 Thank  you both, I used it before but not on the XD and the menu has drastically changed.. Looks like a PSP now.