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Best and cheapest are often mutually exclusive. This issue also depends on how many persons travel together? I'd try to get one of the discounted train fares available through the web, check on: www.bahn.de or www.oebb.at
We go to Westendorf a lot and have found organising a taxi/bus the best way to get there from Munich. There are also buses organised to meet the flights from Ireland, so it is worth contacting to find out wahts available. Here is the link. westendorf.net/westendorftaxi/en/home.html
We just did this and we got the train from Munich central station to Wörgl, and then from there to Westendorf. Make sure you book a ticket and get on a train to Westendorf, Tirol and not Westendorf in Germany as we did the first time!