This character is a Other Symbol and is commonly used, that is, in no specific script.
The glyph is not a composition. It has no designated width in East Asian texts. In bidirectional text it acts as Other Neutral. When changing direction it is not mirrored. The word that U+1F548 forms with similar adjacent characters prevents a line break inside it.
The Wikipedia has the following information about this codepoint:
The Celtic cross ( unicode: U+1F548 π) is a form of Christian cross featuring a nimbus or ring that emerged in Ireland, France and Great Britain in the Early Middle Ages. A type of ringed cross, it became widespread through its use in the stone high crosses erected across the islands, especially in regions evangelised by Irish missionaries, from the ninth through the 12th centuries.
A staple of Insular art, the Celtic cross is essentially a Latin cross with a nimbus surrounding the intersection of the arms and stem. Scholars have debated its exact origins, but it is related to earlier crosses featuring rings. The form gained new popularity during the Celtic Revival of the 19th century; the name "Celtic cross" is a convention dating from that time. The shape, usually decorated with interlace and other motifs from Insular art, became popular for funerary monuments and other uses, and has remained so, spreading well beyond Ireland.