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*The Dictionary of Scottish Place Names and the elements that go to make them up* gives:  
Allt (also ‘uillt’): a stream; burn or brook  
-shellach (as a suffix): anglicised form of ‘seilach’, a willow tree

No example has been found in any gazetteer of a stream or burn of this name. The only well-known use of the name other than the dance is Alltshellach House in Onich, near Fort William. This was built in the 1850s as the residence of the Episcopal Bishop of Argyll and the Isles and is now a well-known hotel.
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