After spending some time without any occupation, Quillinan enlisted in the army as a cornet in a cavalry regiment stationed at Walcheren. Some time afterwards, he passed into another regiment, stationed at Canterbury. A satirical pamphlet in verse, "The Ball Room Votaries", involved him in a series of duels, and compelled him to exchange into the 3rd Dragoon Guards, with which he served through the latter portion of the Peninsular War. In 1814 he made his first serious essay in poetry by publishing "Dunluce Castle, a Poem", which he followed with "Stanzas by the author of Dunluce Castle" (1814), and "The Sacrifice of Isabel" (a more important effort in 1816), and "Elegiac Verses" (1817), addressed to Lady Brydges in memory of her son, Grey Matthew Brydges.