According to Cassidy, this word comes from the Irish aitheach, meaning a churl. Aitheach could be pronounced ay-hakh or ahakh. It doesn’t sound much like the word hick and it is a very old-fashioned, literary word in Irish. Ó Dónaill’s dictionary gives tuaitín, farcach or geoiste as words for bumpkin or country person. The fact that it translates aitheach as ‘churl’ is perhaps an indication that it is not a modern expression.
Back in the real world, the dictionaries are united in regarding the word hick as being a pet form of Richard which must have been associated with country communities. Hick is first recorded in the sense of a country bumpkin in the year 1565, which is far too early for it to have a convincing origin in Irish and it certainly disqualifies it from being anything to do with American slang.