Irregular Fence Problem

I`m looking for ideas on how to get round this problem.
I have a very old Cornish Wall ( i live in Cornwall) along one side of my garden `built` of chunks of granite and large stones "set" in soil. Its about 60cm high and 10mtr long seperating our garden from our neighbours.
I want to put fence posts up along the edge and erect a ranch type fence with the boards starting at the top of the wall upwards.
Problem I have is the wall is irregular along its 10mtr length doglegging in and out aprox 30-40cm from a straight line at its worst point and if I use normal 10 x 10cm square posts the rails won`t run straight from one post to the next.
Round posts may be an answer but i`m not sure if the rails would get enough support surface on them.
I`d be very grateful for any other ideas or advice.

Not a 'proper' wall? No proper foundation?

Either dig holes up against the wall and concrete the posts in one side, or the other.

Or (P.I.T.A. method), remove sections of the wall, dig hole in centre of the wall, concrete posts in, rebuild wall around base of post. You could either get a specialist in to do the wall, or carefully document, number and photograph each major part of the wall as you remove it, Time Team style! Then you'll know where everything went.

To match the wall, use rustic irregular fencing rails like this


There's a fence like this near to where I live, and the rails really are all over the place like giant bananas. Actually, they are much longer than in the pic, so require less posts

Thanks for the help to those who read this post.
Not being too sure on this I have decided to take out the parts of the wall on my side that restrict having the fence in a straight line so i can use 4" sq posts in line. Will then rebuild the wall around post base.
Hope I can get down deep enough for the posts, 2.7m with 2.0m above ground.

I'd get one of these and a hand trowel. And kneepads!

Now the (half) decent weather is here i`m digging the post holes but having to take out large rocks that are under the surface leaving very large holes up to 40 x 40 cm wide, depth is ok at 70cm. The earth is very fine and loose as well ( the local cornish folk say the wall is some 200years old).
I would have used premixed postfix for the posts but as the holes are so large i thought this would be very expensive so i`ve bought bags of B&Q premixed ballast to mix with cement.
Can anyone tell me the ratio of cement to ballast i should use for this size hole to get a solid fixing and am i ok to use a dry mix for filling as advised elsewhere on other postings for postfix ?
I can`t find info elsewhere on this site for mixing your own concrete for posts, only for premixed postfix.
Sorry this may seem a long posting but i don`t know any other way of getting across my message.
Grateful for any advice. For some reason had PC problems connecting to the web last few days so please be patient if i dont respond quickly to any replies.
Thanks.
Mike

you should be using 1:6 ratio. much cheaper to use normal balast and mix it yourself. totally fine to put a dry mx in. Ram it had as you put it in.