Thankyou Gadget and hmcc for your replies. The McKenzies (Hugh and Bridget) were farmers at Glentoo which is in the Kirkcudbrightshire, although some of their 7 sons were registered in Balmaghie.
One of the sons, Kenneth (my 2xgreat grandfather) married Catharine Burgess in Urr in 1829 and I have noticed that a John McKenzie married Grace McNaughton also in Urr at almost the same time. I'm not sure if he was one of the brothers, but is more than likely.
I note that hmmc mentioned that Redcastle was also in Urr.
Hugh and the remaining four sons went to New Brunswick and I have records of a Petition for Land there in 1827 which ties in well with his details.
John was drowned in a shipwreck in 1831 off the coast of Nova Scotia.
Hugh and the last three sons, Hugh, Kenneth and Edward built a boat called the 'Brothers' and sailed to Albany, Western Australia in 1840.
I am writing a story about the McKenzie brothers, hence my interest in what happened to William, George and Cuthbert who have the monument in the Kirkcudbright cemetery.