Have you had any replies to this query ? It is strange how things sometimes fall into place. How close to 1854 do you need ? I know The Prince of Wales Volunteers were in Devonport in 1849 as I was doing a search, while I was in the library today, of a family from the East Coast/Midlands area of Ireland and spotted this piece of information.
Try a Google search for Devonport Garrison Regiments 1854 and Athlone Garrison Regiments 1864. Also do a search for Athlone Garrison and you will discover that prior to 1899 the west of the town was in Co. Roscommon and the east of the town was in Co. Westmeath.