I once had a load of redbull cans that i had to use up and for some reason was taken with the idea of topping up a pint of caffreys (i'd had one mouthful) and it was quite nice as i remember. Now i know caffreys is a bitter/stout and obviously guinness is quite a bit thicker and more characteristic in taste, but it might work? Let me know if you try it.
I agree with Melliexr aboput the blackthorn, but even the local winos might not like it too much - pour the revolting stuff down the drain and get some decent cider!
Had a few nasty experiences on Blackthorn in the past, and it just tastes like cheese to me now!
Guinnes is nice with a shot of Baileys in, so I am told.