The whole ogham alphabet is a family and a number. There are four families and five counts, and that gives twenty letters.
How the letter is formed
Dair is the second letter of the second family, Aicme hUatha, and is cut as two strokes on the second family's side of the line, opposite the side used by the first family. On a stone, that means a reader has to know which edge is being read from before the families can be told apart at all, since the two sides of an arris are not distinguishable in the abstract.
Why the mirror matters
Aicme Beithe and Aicme hUatha use identical strokes on opposite sides of the same line. A two-stroke character is Luis on one side and Dair on the other. If the reading direction of an inscription is misjudged, every letter of the first two families turns into its counterpart in the other, and the text becomes a different text rather than an unreadable one. This is a known hazard in the epigraphy and one reason inscriptions are normally read upward along the left arris by convention.
The sound
Dair carries the value D and is common in the surviving corpus of personal names, cut in Ireland and in Irish-settled parts of Britain between roughly the fourth and sixth centuries.
The oak attached to this letter is a manuscript gloss, several centuries later than the stones, and is recorded separately.