raven

Possibly a telegraph cable marker pole. This Facebook image of its twin on Cable Road seems to confirm the Whitehead Yacht Club one in your pic: https://www.facebook.com/photo/?fbid=1318422055384161 Note the comment: "There was a second tall mast at the cable hut beside the yacht club. If you were at sea and lined them up, it would indicate the direction on the undersea cable of the sea bed and to avoid anchoring near it." Looking at Google Maps, the twin in the photo appears to have been at the junction of Cable Road and Balmoral Avenue but is no longer there. This one is on the Bull Wall in Dublin, though the font is different. Unfortunately no indication of the original colours of the pole or sign. https://maps.app.goo.gl/4cdsXZH2HELLm2g78