Well here's a clue, it's a legal British hallmark that was introduced in, I think, 1983, but it would not be legal to purchase an item with this hallmark from a shop in the UK.
Well done, I believe these marks were brought in following the introduction of Convention hallmarking, whereas the exporting country would hallmark the item and there would be no requirement on the importing country to hallmark the piece. The UK was then faced with the problem of creating new hallmarks if producers were supplying to countries that used a lower standard of silver than that required by law in the UK.
These images came from the Irish Government website below: