Pangur Ban means "white foller" in Gaelic. Ban, as others have mentioned, means white and pangur means fuller. A fuller is a person who full cloth. In this case, the monk who originally composed the poem Pangur Ban about his cat probably was thinking of the way that cats clean themselves and that it resembles working with cloth. The poem itself is in many books of medieval Irish poetry, but I highly recommend an older collection edited and annotated by Thomas Kinsella. The poem itself is a contemplation and comparison of the monk's scholarly pursuits with the hunting pursuits of his pet cat.