In Cork City marathon I ran a leg in the relay event. Love it!
What other marathons in Ireland or the UK have the relay included?
Belfast and Limerick do a relay as well as part of the marathon. Both have been held already this year, though.
Manchester had a 2 x half and 4 x quarter option.
For a "not a marathon distance but may as well be" there is Man v Horse in Wales which was on today and has a relay team event. Well worth doing if you can figure out how to get there.