Complex sentence: Major George O'Malley started his military career as a volunteer in the Castlebar Yeomanry and was present at the Battle of Castlebar, then served in the North Mayo Militia before joining the 13th Foot.
Split into simpler sentences: Major George O'Malley started his military career as a volunteer in the Castlebar Yeomanry and was present at the Battle of Castlebar. He then served in the North Mayo Militia before joining the 13th Foot.