Quirinus is the god of the Roman slate, and one of the original Capitoline Triad, now called Archiac Triad. He was also an epitet of Janus, as in Janus Quirinus.
His name is deprived from the word quiris which means "Spear". All together, his name is probably an adjective meaning "wielder of the spear".
Quirinus was probably a Sabine god of war. According to wikipedia, "The Sabines had a settlement near the eventual site of Rome, and erected an altar to Quirinus on theCollis Quirinalis, the Quirinal Hill, one of the Seven Hills of Rome. When the Romans settled there, they absorbed the cult of Quirinus into their early belief system — previous to direct Greek influence — and by the end of the first century BC Quirinus was considered to be the deified Romulus".
He became an important figure in early Rome, a part of the first Capitoline Triad along with Jupiter and Mars, who was then an argiculture god. Marcus Terentius Varro notes Capitolium Vetus an earlier cult sited on the Quirinal, devoted to Jupiter, Juno and Minerva, among whom Martial makes a distinction between the "old Jupiter" and the "new".
In later times he became less important, losing his place to the more well-known Capitoline Triad (with him and Mars being replaced by Juno and Minerva). Later on, Romans began to drift away from the state system to more personal and mystical cults (such as those of Bacchus). In the end, he was worshiped almost exclusively by his flamen, the Flamen Quirinalis, who remained, however, one of the patrician flamines maiores, the "greater flamens" who preceded the Pontifex Maximus in precedence
In early Roman times, he was decipted as a bearded man wearing military and religious clothes.
His festival is the Quirinalia, which is held on Feburary 17.