Quite a few European languages share what is essentially the same word for willow; sallow or sally in English, probably derived from Old English salh (Anglian) or sealh, seales (West Saxon); salix in Latin; seileach in Scottish Gaelic; saule in French, and so on. In the context of Yeats' poem, sall[e]y is more likely to be an Anglicisation of the Irish Gaelic word than a borrowing of the English word; but it's a fine distinction.

Relationships between The Water is Wide and Carrickfergus have been discussed at some length in previous threads; the "Digitrad and Forum Search" on the main Forum page will find these for you if you ask it.