Hi,
I live in Luxembourg.
I think The Mullertharl region is known as Little Switzerland...and the center of that area is the town of Ecternach. If it's in Ecternach, or every close, then bus 110 and bus 111 go to Ecternach from Luxembourg City.
Yes there are buses, but unless the hotel is in Ecternach or a big town like that, you will be relying on taxis to get everywhere.
Probably, if relying on public transport only, you need to get a taxi to the Gare, or bus 16 to the Gare, then take a bus going in your direction.
I think you will need to call the hotel, and find out how close it is to Ecternach, or Mersch ... then you can plan a route from Luxembourg to Ecternach or Mersch, followed by another bus if necessary and possible, or a taxi.
If you are working in Luxembourg City, then really I suggest that you cancel the hotel in Mullertharl unless it is right in the center of Ecternach ...
Then you can try to book a hotel in Luxembourg and make a trip on the Saturday up to the Ardennes for some hiking, maybe in the Osling area or around Borscheid Castle...
You will have a fantastic day's walking in that area!
Alternatively, you could take the train from Luxembourg City on Friday night, stay in the Victor Hugo Hotel overnight in Vianden, and after looking around Vianden on Saturday morning, take bus 663 to Clervaux to visit the Family of Man (the world famous UNESCO Heritage exhibition) and the Battle of the Ardennes museum at the chateau there, and get the train back to Luxembourg city on Sunday morning for your flight.
There are hundreds of walks you can do, and Luxembourg has the highest density of nature walks in Europe. So you could stay out of site of a town for a day with no trouble! I've hiked all over the country myself, but I'd still recommend you take the Vianden and Clervaux option !
I have put some sites below to help you out, plus you can mail me if you want more help or advice
Hope that helps
Philip
Edit (further to your email):
Hi,
My advice is to forget the Mullartharl and either stay in Vianden on Friday night, and in Clervaux on Saturday night...
Clervaux Hotel:
http://www.hoteldesnations.lu/en/
Vianden Hotel:
http://www.hotelvictorhugo.lu/english/in…
Or, take the train to Kautenbach (50 minutes from Lux-city) on Friday night and stay in the Hotel Hatz
http://www.hotel-hatz.lu/
Tel: (+352) 95.85.61
Then you can do some walks, like the walk up over the hills to Goebleshmule (10 kms) where you get the train back, or carry on to Michelau via Borscheid Castle ...
Then it's easy to get he train back on Sunday morning to Lux-City, and jump on Bus 16 to the airport for your flight