Generally if a soldier served in a garrison battalion, he would have either served in the army of reserve. The army of reserve meant that the soldier could only serve in the UK, Scotland or Ireland unless they volunteered for a line regiment.
Sometimes old, sick or incapacitated soldiers were moved to a Garrison or veterans battalion. Have you tried searching for a pensioners record at the Archives? Most garrison Battalions served in Ireland at some time during the period of the Napoleon wars. I have an ancestor who served in the5th Garrison Battalion, i also found that the 2nd battalion of the 26th Regiment (cameronians) and the 42nd Regiment( Black Watch) transferred soldiers into this battalion. It might be worthwhile looking into the history of some of the line regiments to see if soldiers were transferred into your garrison battalions..