Christoph Stoepel's website records only three Lahers in the German telephone directory, but 115 in the Austrian. As a German name I'd guess that it's a variant of the more common Lahr, which I have assumed came from a German place name, but the Oxford Names Companion derives from the given name Hilary.
http://www.nationaltrustnames.org.uk/Surnames.aspxidentifies Laher, in Britain at least, as "Middle Eastern", and states that the majority of the 300+ bearers of this surname in Britain have Muslim given names. Others have Turkish, North African, Hindi or East Asian names.