Thomas Plunket was born in 1785 in Newtown, Wexford, Ireland. He joined the 95th Rifles in May 1805. In 1807, he took part in the British invasions of the River Plate (1806-1807). During the 2nd Battle of Buenos Aires, the 95th Rifles were heavily engaged in street-fighting during which Plunket killed around 20 Spanish troops while sniping from a rooftop with others from his unit. They retreated when Spanish artillery bombarded their position with grapeshot. Plunket also shot a Spanish officer, who was waving a white handkerchief with the possible intention of inviting a truce. This resulted in further Spanish artillery bombardment which brought about the British surrender.