There is a retaining wall under my house that appears to be acting as a dam, with a moderate pressure of water building up behind it.
I want to get holes drilled into the bottom of the wall, and ag-pipe installed so it will drain properly, but there is only about 3' (90cm) access.
Can anyone suggest what sort of solutions exist for this sort of drilling (BTW, the wall is mudstone, so standard concrete drills don't work well.)

Don't know what you mean by a "standard" concrete drill but I have a Dewalt SDS hammer drill and I promise you it will go through your wall. I have drilled 1" holes in 4" solid concrete walls in just a couple of minutes. This may not be your best solution but it will sure put a hole in the wall.

Ooops, sorry, I should have said - I'm wanting at minimum a 70mm (2 3/4") hole, and by preference 90mm (3.5") or 100mm (4") hole - I can't get a masonry bit that diameter. By standard conrete drill, I was referring to a core drill - the sort that cuts a tubular hole.

Yep, big difference. Not sure what your best approach would be, but a wet core, diamond drill would be my approach.

Is this water being retained inside or outside?

What is the 3' span to work in?