William Butler Yeats was a Protestant. He was a member of the Church of Ireland, which is an autonomous province of the Anglican Communion. Technically, Beckett was indeed an Anglican, although it was common for members of the Church of Ireland to be called "Protestants" rather than Anglicans - thus distinguishing them from the Catholics of Ireland and also from the Anglicans of England.
William Butler Yeats was also a strong believer in astrology, mysticism and the occult. Around 1886 Yeats joined the Theosophical Society, but he was later asked to resign.
From: "William Butler Yeats" on "Books and Authors" website (http://www.kirjasto.sci.fi/wbyeats.htm; viewed 24 October 2005):
William Butler Yeats was born in Dublin into an Irish Protestant family. His father, John Butler Yeats, a clergyman's son, was a lawyer turned to an Irish Pre-Raphaelite painter. Yeats's mother, Susan Pollexfen, came from a wealthy family - the Pollexfens had a prosperous milling and shipping business. His early years Yeats spent in London and Slingo, a beautiful county on the west coast of Ireland, where his mother had grown and which he later depicted in his poems. In 1881 the family returned to Dublin. While studying at the Metropolitan School of Art, Yeats met there the poet, dramatist, and painter George Russell (1867-1935). He was interested in mysticism, and his search inspired also Yeats. This was a surprise to his father who had tried to raise his son without encouraging him to ponder with such questions. Reincarnation, communication with the dead, mediums, supernatural systems and Oriental mysticism fascinated Yeats through his life. In 1886 Yeats formed the Dublin Lodge of the Hermetic Society and took the magical name Daemon est Deus Inversus. The occult order also attracted Aleister Crowley.
As a writer Yeats made his debut in 1885, when he published his first poems in The Dublin University Review. In 1887 the family returned to Bedford Park, and Yeats devoted himself to writing. He visited Mme Blavatsky, the famous occultist, and joined the Esoteric Section of the Theosophical Society, but was later asked to resign...
Through Maud's influence Yeats joined the revolutionary organization Irish Republican Brotherhood. Maud had devoted herself to political struggle but Yeats viewed with suspicion her world full of intrigues. He was more interested in folktales as a part of an exploration of national heritage and for the revival of Celtic identity. His study with George Russell and Douglas Hyde of Irish legends and tales was published in 1888 under the name Fairy and Folk Tales of the Irish Peasantry.
...At the age of 52, in 1917, [Yeats] married Georgie Hyde-Lee, who was 26... the marriage was happy and they had a son and a daughter... During their honeymoon Yeats's wife demonstrated her gift for automatic writing. Their collaborative notebooks formed the basis of A VISION (1925), a book of marriage therapy spiced with occultism... ....At the start of the war  Yeats went to Oxford, but then returned to Dublin, becoming a Senator in the same year. As a politician Yeats defended Protestant interests and took pro-Treaty stance against Republicans.