This is another claim made by the con-man Daniel Cassidy in his terrible book How The Irish Invented Slang, that the slang term welt, as in ‘to welt someone’ (to hit them hard) comes from Irish bhuailte. Cassidy furnishes us with an example of bhuailte in use, which will show any competent Irish speaker that Cassidy knew nothing about the grammar of the Irish language. Cassidy claims that ‘That man is beaten’ would be expressed in Irish as Tá an fear sin bhuailte. In reality, this would not be bhuailte but buailte. There is no reason to lenite the word buailte and without the lenition, it is pronounced boolcha, which is hardly likely to be the origin of welt, is it?
Some expert! He couldn’t even construct a simple sentence like Tá an fear sin buailte without getting it wrong!