Per House of Names: "The surname Cretney was first found in Galloway (Gaelic: Gall-ghaidhealaibh), an area of southwestern Scotland, now part of the Council Area of Dumfries and Galloway, that formerly consisted of the counties of Wigtown (West Galloway) and Kirkcudbright (East Galloway), where they held a family seat. In a strange convolution of heritage and translation from the Gaelic, this name, Cretney is descended from MacBratney, or MacBhreatnaich, the Gaelic, meaning a son of the Strathclyde Briton, or children of the Britons, who settled amongst the Gaels. From their home lands in Clontag and Knockane in Galloway in Western Scotland they descended to Martin Birty who appears in records in 1471. They were known as the Clann a'Bhreatannich, and were originally from the Island of Gigha off Kintyre, a branch of the Galbraiths as early as 1230. The name evolved to Makbretny, and thence to Vretny and Cretny." 
There is a single Cretney who has tested under BY3368, and he is showing a different haplogroup stream than that of other Cretney Y DNA test takers. The others, in fact, are all subclade descendants of BY183076, which, traces to S443, which is defined by Britain's DNA as "Norse Viking. So, the Cretney who has tested to BY3368 shows an NPE at some point in the family tree... likely in either Isle of Man or Galloway, Scotland.