Britt,
You haven’t said when any of these people were born, nor what denomination they were. However if Andrew served in the 1812 war, then I assume they were all born in the 1700s. Researching in the 1700s in Ireland is notoriously difficult due to a general lack of records. It may not be possible to find out much about them, especially if you do not know exactly where in Armagh they lived.
As to fleeing Scotland because of persecution, and changing their surname, I am not sure how you would verify that. There was a huge migration from Scotland to Ireland in the 1600s. Some came in the early 1600s as part of the Plantation of Ulster, others followed throughout that century due to a variety of factors (mostly economic) and in the 1690s there was a big intake due to famine in Scotland. Probably something like 200,000 Scots settled in Ireland in the 1600s (representing something like 20% of the entire Scottish population). Many settled in Co. Armagh. The vast majority were Presbyterians (ie Protestant). Some of those were Covenanters fleeing persecution in the 1680s. That period was known as the killing times.
The Stewarts were Catholic and most, though not all, of their supporters were also Catholic. So the Kean family denomination may give a clue as to whether they were possibly Stewart supporters.
Possibly DNA testing may be a way of matching with others who have additional information about where the family originate. Family Tree DNA reportedly has more people with Ulster roots than any other company. That obviously increases the chances of finding a match. You might want to try them or, if you have already tested, you can transfer your results to them for no fee.
The North of Ireland Family History Society is running an Ulster DNA project in conjunction with FTDNA and can offer testing kits at a reduced price. http://www.nifhs.org (Go to DNA project on the website).
