This is a gesture known as the Christogram, and is considered the original “sign of the cross.” The fingers are positioned to form the Greek letters ICXC, an abbreviation of the Greek name of Christ: IHCOYC XRICTOC. This gesture is ubiquitous in Renaissance images of Christ and the apostles, as well as in portraits of Saints and clergy.
The Christogram is used today as a traditional gesture of blessing by priests in the Eastern Orthodox Church.
Curiously, the same gesture is known in Hindu and Buddhist traditions as the prana mudra, a symbol of healing.
Hans Memling, Christ blessing
The Celtic Ogham (pr: Oh-yam) alphabet dates from the fourth century. The alphabet is named for Ogmos, the Celtic god of knowledge and communication. Ogmos was associated with the Gaulish god Ogmios, and the Greek Hermes.
The ogham alphabet has twenty letters, each named for a different tree sacred to the Celtic Druids. Each letter is made up of one to five straight or angled lines incised on a straight base line. Because the number of letters, and the number of lines that make up each letter, some scholars have theorized that the ogham may have originated as a system of hand signs, likely a system of communication used by the Bard class.
Examples of Ogham writing have been found all over the British Isles, and even as far away as Spain and Portugal. All surviving examples exist as stone carvings, usually on tombstones and road markers. Although it is commonly used by modern Druids and other NeoPagans as a divination system, there is no real relationship between modern and historical divination systems.
When used as a divination method, the letters are usually notched into straight twigs and used much like runes. To see the entire Ogham alphabet and download a set of Ogham cards, see: The Celtic Oracular Alphabet