"Dominie Deasy kens them a'": Stephen remembers the various kinds of shells that his employer keeps "heaped in the cold stone mortar" on the table in his office. "Dominie" is a Scottish term for a schoolmaster, often one in the Church of Scotland, and "kens them a'" is Scots dialect for "knows them all." So, Schoolmaster Deasy knows all the types of sea shells.
Which begs the question of why Stephen may be lapsing into Scottish idiom at this moment. The likely explanation is that Deasy is an Ulsterman, and much of Ulster was settled by Scottish emigrants in the 17th century. As a Unionist from the North, embued with the Protestant ethos of making and saving money, Deasy figures in Stephen's consciousness as a Scotsman.
Ellmann identifies the real-life model model for Mr. Deasy, Francis Irwin, as an "Ulster Scot, very pro-British," and he observes that the name Deasy is "oddly inappropriate for an Ulster Scot" (33). But Deasy is a common name in Angus county in Scotland, tracing back to the Picts, and some Deasys did move to Ireland.