The Shenkottai to Achankovil road is far better for driving compared to the Punalur to Achankovil route via Mullumala .
The route is like this: Shenkottai --> Panpozhi --> Mekkarai --> Kottaivasal --> Kumbhavurutty --> Achankovil (25km)
At Panpozhi, ask for direction to Achankoil. At the outskirt of the village you will reach a bifurcation, the left goes to a hilltop shrine called Malaikoil. Take the right fork for Achankovil. After this there are less chance for a confusion as you follow the main route to Achankovil.
After Mekkarai village, the road enters the forest, and shortly after the ghat starts . In about 6 to 10 km you'll pass the ghat and those 23 hairpin bends. Towards the end of the climb is the shrine of Karuppannasamy at Kottaivasal.
This shrine is at a hairpin bend and overlooks the valley beneath. At this point you are almost a kilometer above the sea level.
It's believed that it's here Lord Ayyappa met Karuppannasamy on his way to Sabarimalai as the final adobe. Karuppannasamy became lord Ayyappa's lieutenant, who guarded the shine of Ayyappa at Sabarimala. You'll see Karuppannasamy's shrine at Achankiol and also at the base of the 18-steps at Sabarimalai.
Further to the end of the ghat section, you'll pass two popular waterfalls,one at Kumbhavurutty and the other at Manalar. Another 5 to 6 km, you'll reach the Achankovil village.
This is an interstate route. There is no entry tax for non-commercial private vehicles. But commercial (like tourist cabs) need interstate permit. Also note there are not much facilities (like hotel, fuel station etc) en route after Shenkottai. Do everything at Shenkottai or Tenkasi.
Those coming by train from places like Madurai, Tiruchchirapali, Chennai, Tirunelveli etc Sengottai (SCT) is a nearest railway station.
Pothigai Express ( Train No:12661/12662 ) between Sengottai (SCT) and Chennai Egmore is the ideal overnight train connection in the Madurai, Tiruchchirapali route. The train reaches Shenkottai in the morning. Hire a cab from Shenkottai for the rest of the 25km journey to Achankovil and back to Shenkottai. Or even try the KSRTC bus (not very frequent) to Achankoil from Shenkottai. You can take the night train for the return journey.