No special access issues
1) Access issues and rules:
Khao Takiab is not part of a national park and has been used recreationally for decades. It is a popular spot for fishing and hiking. As such, there is generally little reason why climbers shouldn't be allowed to climb there - and the resident monks generally are not opposed to bouldering.
Nevertheless, climbing in Thailand has a troubled past, with many areas getting closed with little to no warning or justification. Keep a low profile, take out any trash you find, eat at the local restaurants and don't get hurt.
2) How to get here:
From Bangkok, take a train (~4 hours), bus (~2.5 hours) or car (~2 h) to Hua Hin. Getting to Khao Takiab from central Hua Hin takes around 15 minutes by taxi or motorbike. If you are staying in Takiab, the neighborhood south of Hua Hin, the boulders are within walking distance.
3) Access to the boulders:
Monkey Mountain: The boulders are directly below Monkey Mountain restaurant and easily accessed from the beach.
Le Mer Left: Drive to Le Mer restaurant. From the parking, climb over the fence to the beach, the boulder with Rare Events should already be visible from the parking. Alternatively you can walk over the rocks from Monkey Mountain.
Le Mer Right: Drive to Le Mer restaurant, but stop in the last curve maybe 100m before the restaurant / right after the exit from the road to the temple. There is a little house in the head of the curve (12°31'04.0"N 99°58'54.1"E) and a steep footpath down to the ocean through the bushes and cacti. Once you've reached the restaurant parking, you're too far. Access from Le Mer Left is difficult because of a steep cliff blocking the way (but possible).
The Harbour: Drive to Khao Takiab Seafood Market. There is a little footpath between the restaurants going right (12°30'57.2"N 99°58'48.1"E). Follow along the ocean. Beware of overly protective dogs. The boulders start shortly after the last house (5-10 minutes walk).
Secret Cove: Walk to and through The Harbour sector, then further over the rocks for another 10 minutes or so. The sector starts at a small sandy cove with a cave. To get to the most promising and least developed area that houses boulders such as "the crack", continue for another 100-200 meters.
It is possible to walk all around the mountain, but not practicable with crashpads.