Google search discovers that either is widely used - consistency is sometimes unattainable. The Roud Index lists 4 occurances of the song using Screen and 6 that use Skreen so it's also inconclusive. I prefer to use Screen in order to emphasise the generally accepted locus. However, the songs first appearance (in Herbert Hughes' Irish Country Songs' vol. 3) uses Skreen and perhaps the matter should be further investigated. According to Roud the Hughes version was collected in Kerry. I wonder has anyone the book - mine is currently inaccessible - to check whether this is correct, it would be easy to type Kerry instead of Derry?