The fantstic work of Chris Baker on the Long Long Trail website has identified 274 instances of brothers who were killed on the same day in the Grear War. And one pair of brothers came from Portsmouth.
Charles James and Goerge Ernest Roberts were both Corporal Signallers serving with the 2nd Battalion of the Royal Munster Fusiliers. Regular Soldiers, they were sent to France on the outbreak of war. They were at garrisoned at Aldershot, and were part of the Guards Brigade in the 1st Division. The Battalion had been in Ireland on garrison duty until 1912. They landed at Le Havre via Southampton on 14 August, and went straight up to the Front.
After the Battle of Le Cateau the British Army retreated. Along with the 15th Hussars the 2nd Munsters fought a stiff rearguard action at Etreux. A single Battalion were facing an entire German Army Corps. In a Rorkes Drift style action the Battalion suffered severe casualties, where they were surrounded and virtually destroyed. The survivors left the front line and became divisional troops.
Charles and George Roberts were both killed on the same day – 27 August 1914 – and are buried in the same grave at Etreux British Cemetery in France.
It was not common – but not unusual – for young British men to join an Irish Regiment. My research suggests that of young men joining the infantry in peacetime, around 50% joined the Hampshire Regiment, and around 50% joined other Regiments. Although the local country Regiment was natually the most obvious choice, perhaps family connections persuaded the Roberts to join the Munsters?