This character is a Other Letter and is mainly used in the Ogham script.
The glyph is not a composition. It has no designated width in East Asian texts. In bidirectional text it is written from left to right. When changing direction it is not mirrored. The word that U+1690 forms with similar adjacent characters prevents a line break inside it.
The Wikipedia has the following information about this codepoint:
Ailm is the Irish name of the sixteenth letter of the Ogham alphabet, ᚐ. Its phonetic value is [a]. The original meaning of the name cannot be established with certainty. The Bríatharogam kennings all refer to the sound [a] and not to the meaning of the letter name, either as the sound of a "groan", or to the Irish vocative particle, á. Thurneysen suggests that Ailm, Beithe was influenced by Alpha, Beta. However, beithe is an Irish word, and there is no reason to consider ailm a sole, loaned letter name among the original feda; Thurneysen did not suggest this letter name involved such a borrowing. The word is attested once outside of the Ogham grammatical texts, in the poem "King Henry and the Hermit",
caine ailmi ardom-peitet
which translates to
Beautiful are the pines which make music for me.
This single reference is the reason ailm is sometimes associated with pines. However, the poem likely post-dates origins of the medieval tradition of arboreal glosses of the ogham letters, so is more probably influenced by this tradition than an independent source for the meaning of ailm.