The sinking of the Lusitania, which occurred during World War I, was a significant event that caused outrage in many countries, including Ireland. The inclusion of the sinking on an Irish recruiting poster likely aimed to invoke anger and a sense of duty among Irish citizens towards the British cause, as the Lusitania was a British ship and the incident contributed to anti-German sentiment. Additionally, it could have been used as a way to emphasize the shared struggle against a common enemy.
