I think you’ll find a subscription to findmypast helpful.
The British India baptism records show several children of Joseph and Sarah Magrath/McGrath and each one gives Joseph’s occupation as Band Master 61st Regiment.
I don’t know enough detail about military music in Victorian times to answer your question accurately but I think that it was unlikely that the Band Master, a senior role, would have been a civilian. It requires a great deal of military knowledge, regimental tradition and organisational experience to provide the range of activities expected of musicians in war and peace. This is above and beyond exceptional musical ability.