Yeats published "Down By The Salley Gardens," a poem based on an Irish folk song, in a collection called Crossways in 1890. Although some readers have tried to locate the "Salley" gardens, the source is the Gaelic word for willow trees, saileach. Willows were important in Irish life because, in rural areas, the willow provides the most common material for thatching, a very common roofing material.
The poem (and the song on which it is based) is a ballad, which is often described as a song or poem that recounts a sad or tragic personal or communal event. Most ballads use what is called ballad form, which calls for quatrains (four line stanzas, either rhymed or unrhymed) that contain alternating four-and-three-stress lines, with the second and fourth lines rhyming. Although "Salley" is a ballad, it is not consistent in following strict ballad meter. Most of Yeats' even-numbered lines are iambic trimeter, three sets of unstressed and stressed syllables, but there are several exceptions. Still, the poem maintains the "Ta-Dum" rhythm of the classic ballad form.
Given the ending of the poem--the speaker is "full of tears"--the setting is appropriate, two lovers surrounded by willows, which are often described as "weeping" because the branches bend downward. The speaker is an older man looking back on his younger and "foolish" self. The refrain at the end of each stanza evokes the sadness of a man looking back on the love he has lost because he could not match his ambition with his lover's view of life:
But I, being young and foolish,/with her would not agree. (Stanza 1)
But I was young and foolish,/and now am full of tears. (Stanza 2)
The first stanza ends with the speaker's rejection of his lover's advice to "take love easy,/as the leaves grow on the tree," but the second stanza ends with the finality of the full realization of what he has lost through his stubborn rejection of his lover's view of life.
The theme of innocence is personified in the speaker's lover, who is described in each stanza as having either "snow-white feet" or a "snow-white hand," both of which are consistent thematically with the advice she tries to give her lover to let love happen naturally--"as the leaves grow on the tree"--and to enjoy the slow unfolding of life "as the grass grows on the weirs," both of which are powerful images of nature and symbolically represent the virtues of a life attuned to nature rather than the ambition that the speaker now recognizes as his greatest mistake.
This poem is about memory and loss and the heartbreaking realization of a life-altering missed opportunity. The natural setting and the purity of the young woman are set against the speaker's stubborn failure to understand, when he was young, that love and ambition are not compatible. His realization arrives too late, but at least it arrives.