Hi Frank,
Welcome to the Forum.
This mark is almost certainly that of Henry Johnston. The hallmarks indicate that they were applied at Dublin in 1911. He entered a similar mark with London Assay Office in 1901.
Henry Johnston's business was extensive, with outlets in Belfast, Dublin, Cork, Londonderry and Newcastle-upon-Tyne. He manufactured walking sticks and umbrellas using Irish Blackthorn.

Johnston - Belfast - 1893

Johnston - Belfast - 1896

Johnston - Dublin - 1900

Johnston - Cork - 1911
Trev.