Pirates refers to various things in UO.
Sometimes, in the days before Trammel, certain Reds would wander the land on boats, attacking other players on boats, and they were referred to as pirates. Some would Roleplay (see Roleplaying the part, and others would not.
Various EM Event on certain shards featured NPC Brigands called pirates. They were also used as part of the Vanguard event in some of the cities. However, in the latter appearance, they were bugged, and could neither damage players nor be damaged directly by players (pets would work under certain circumstances).