Ishak Pasa Palace is in Dogubayazit.
The best and most efficient way is to fly, either to Van or Kars then take the bus to Dogubayazit.
THY
http://www.thy.com/en-INT/index.aspx and Pegasus Airlines
https://book.flypgs.com/Common/MemberRez… have daily flights to Van. Only THY flies to Kars, Pegasus doesn’t.
My recent experience was that I flew to Van then took the bus to Dogubayazit takes about 3 hours. I flew from Kars back to Istanbul.
Ishak Pasa Sarayı (Palace) is just outside of Dogubayazit (5 km or so), small minibuses go from the bus terminal.
The second best option would be taking the train to either Tatvan or Kars from Istanbul. If you chose this option, which is more economical than flying but obviously takes longer. If I were to elevate this option I’d take the train to Kars after bus it to Dogubayazit. That is about 4 hours bus journey goes via Igdir. When you look at the schedule below takes about 1.5 day or so by train to Kars and plus a bus ride to get to Dogubeyazit.
http://www.seat61.com/Turkey2.htm scroll down to the middle of the page see the schedules and fares.
The other option is taking the bus from Istanbul to Dogu Beyazit but it could be very tedious and aggravating travelling from one end of Turkey to the other on the bus. However, if you want to take this option, you need to phone the bus company to get information on the schedules and fares
http://www.otogaristanbul.com/index.php?…
Ishak Pasa Palace is awe-inspiringly amazing…You are going to love it. It was undergoing restoration when I was there (July 09) - they were building a roof over it. Surely, one of the best things to see in Eastern Turkey.
Have a good time.