Don't plant an ivy, withn 15 years the hedge will be dead or very poorly, as it sounds like you have a hawthorn, or something similar, underplant with holly, it will cope with the shade nicely and will grow through it slowly. Evergreen so will close the gaps, will take about 6-8 years to get up 6ft though.
P.S. make sure that you water the holly for the first 2-3 years in the summer, as the mature hedge will suck up all the water from its immature roots.