Ian there are 4 Mack births in Mayo, Westport father Sgt George Mack of the 4th Mayo Staff, in the other 3 he is just George, Children to mother Catherine are George; James; Jane and WIlliam.
period 1821 to 1831. There is no sign of a marriage record.
The milita were formed by those loyal to the King which gave them the right to bear arms etc, the local one where I lived had two comings and even went overseas. I would try the Mayo library for information on the militia, there may be a book or some information, look at the local history section of the site.
If you are on Facebook there is an excellent private Facebook page called the Royal Irish Constabulary 1816 to 1922 A Forgotten Irish Police Force, it is well run and apply to join, the knowledge is staggering and one of the members has written the history of the RIC.