Luis is the second letter of the Ogham

Ogham

Ogham is an Early Medieval alphabet used primarily to write the Old Irish language, and occasionally the Brythonic language. Ogham is sometimes called the "Celtic Tree Alphabet", based on a High Medieval Bríatharogam tradition ascribing names of trees to the individual letters.There are roughly...

 alphabet, derived either from luise "flame

Fire

Fire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....

" or from lus "herb

Herb

Except in botanical usage, an herb is "any plant with leaves, seeds, or flowers used for flavoring, food, medicine, or perfume" or "a part of such a plant as used in cooking"...

". Its Proto-Indo-European

Proto-Indo-European language

The Proto-Indo-European language is the reconstructed common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans...

 root was either * 'to shine' or * 'to grow'. Its phonetic value is [l].

The Auraicept na n-Éces

Auraicept na n-Éces

Auraicept na n-Éces is claimed as a 7th century work of Irish grammarians, written by a scholar named Longarad....

glosses the name as cairtheand "mountain-ash", i.e. "rowan

Rowan

The rowans or mountain-ashes are shrubs or small trees in genus Sorbus of family Rosaceae. They are native throughout the cool temperate regions of the Northern Hemisphere, with the highest species diversity in the mountains of western China and the Himalaya, where numerous apomictic microspecies...

" (Modern Irish caorthann). The associated verse is

Li sula "lustre of eye"

The Auraincept interprets this as "delightful to the eye is luis, i.e. rowan, owing to the beauty of its berries".