The letter Ꞅ (minuscule: ꞅ) is an Insular script form of the letter S. The uppercase is encoded in Unicode at U+A784 https://www.charbase.com/a784-unicode-latin-capital-letter-insular-s, and the lowercase is encoded at U+A785 https://charbase.com/a785-unicode-latin-small-letter-insular-s.
It should not be confused with the long-legged r,, which is an obsolete IPA symbol.