One use was as a the missile in a giant catapult:
"BEER BARREL SHELLS
Incredible Story of Austrian Engine of War
According to a private message from Berlin the Austrians in their attack on the Servian positions outside Belgrade used a remarkable engine of war for the first time.
This machine is constructed on the principle of the ancient Roman catapult, and throws barrels filled with stones and explosives at a high trajectory into the enemy's position with terrible effect, the stones being split up into thousands of pieces and hurled over a wide area.
The range of these machines is necessariiy small but for the warfare on the Danube they have proved peculiarly effective. The message adds that numerous transports are coming up the Danube towards Belgrade loaded with empty beer barrels for the new engine."
Western Times - Friday 20 November 1914, page 8.
That is just weird and, to be honest, a great waste of such a lovingly crafted object as a beer barrel. It sounds a bit primitive, in the days of heavy howitzers, to be messing around with catapults.
This sounds more reasonable: using barrels as a transportation device.
The Excelsior's Rome correspondent says: "According a telegram from Lugano, the police are seeking for several mysterious individuals of German nationality who for some days have been buying old rifles from the inhabitants of Lugano. They paid fairly high prices for them. According information received, these rifles were intended for the Tripolitan rebels. Following the discovery made at Venice of 92 casks of beer sent from Germany to Tripoli, advices from Tripoli state that the police have arrested the consignee of these famous casks. His identity has not been revealed. Further arrests are expected among the Germans and Austrians who have remained in considerable numbers in Tripoli."— Reuter."
Exeter and Plymouth Gazette - Monday 05 April 1915, page 6.
I assume the barrels were filled with guns, though frustratingly the article doesn't make that totally clear. Unlike this one:
RUMANIA STOPS GERMAN CONSIGNMENTS.
Reuter's Agency is informed with regard to the renewed efforts which are being put forward by Germany to induce Rumamia to permit the passage of arms to Turkey, that the Rumanian Government continues successfully and energetically to prevent all arms from Germany and Austria, intended for Turkey or elsewhere, from passing through Rumanian territory.
In addition to the stopping already reported of double-partitioned waggons, the Rumanian authorities have lately confiscated trains laden with what were represented to be barrels of Munich beer. In order that this might be sent forward in good condition, the German authorities had telegraphic messages sent to various stations for supplies of ice blocks to be provided. Suspicion, however, was aroused, and on examination the barrels of Munich beer were found to contain ammunition. All were promptly seized.
Subsequently large consignments of enormous blocks of cement, consigned to Constantinople from Germany for building purposes, attracted attention. On examination these building blocks were found to be hollow, and filled with shell cases for large German howitzers. All have been stopped and confiscated. "
Liverpool Echo - Wednesday 14 July 1915, page 5.
Personally, I'd be dead disappointed if I found my barrels of Munich Beer only contained bloody bullets. Not much fun drinking those. Roumania eventually joined the war on the allied side.