The measure varied in size according to the quality and situation of the land; typical values were between 60 and 120 Irish acres (39.33 and 78.66 hectares)..
Origin
Mid 16th century; earliest use found in Parliamentary Papers. From Irish baile bó (now hist.) from baile place, town, townland, unit of land measurement + bó, perhaps with reference to the amount of land needed to graze a certain number of cows.