Shirriffs are the closest one comes to policemen in the Shire. They had no uniforms aside from a feather in their caps, and acted under the command of the First Shirriff, who was also the Mayor of Michel Delving. As a general rule there were only twelve in the entire Shire, with three assigned to each Farthing. It wasn’t a particularly difficult job, described by one Shirriff as “walking round the country and seeing fol, and hearing the news, and knowing where the good beer was.” Their duties also occasionally included helping the Shirefolk with animals gone astray.
During the War of the Ring, things changed somewhat once Sharkey and Lotho Baggins came to power. The ranks of the Shirriffs were greatly increased until there were hundreds, with Shirriff leaders being appointed and distinguished by a second feather in their hat. The Shirriffs were armed with staves, and permanent barracks called Shirriff-houses were built (or converted, often out of inns). Their new duties were to enforce the Chief’s Rules, often by dragging troublemakers to the Michel Delving storage-tunnels, which became known as the Lockholes. The Rules also forbade Shirriffs from quitting their jobs. During the Scouring of the Shire, most of the Shirriffs joined their kin in fighting the Ruffians, although a few chose to quietly disappear into the night instead.
The only named Shirriff is one Robin Smallburrow, from Hobbiton. A friend of Samwise Gamgee, he was a member of the contingent sent to arrest Frodo Baggins & Co. for Gate-breaking, Tearing up of Rules, Assaulting Gate-keepers, Trespassing, Sleeping in Shire-buildings without Leave, and Bribing Guards with Food. The attempted arrest was unsuccessful, to put it mildly.