This cancel is causing me some difficulty. It seems to be #187, but there were two post offices in England that were allocated that number, Consett and Staveley. I don't know if there's a way to tell the difference between the cancels used by the respective post offices and I can't find any examples of either online to use for a comparison.
In any case, this cancel has a diamond shape, which means it is almost certainly not English. The Irish numeral cancels were diamond-shaped, which could make it Dundalk. However, I have been able to locate several examples of the Dundalk cancel online, and the diamond shape is much wider than this one appears to be. This is, by comparison, rather longer from top to bottom than it is wide. Judging from the Irish numeral cancels I've been able to find online, no Irish cancel had this particular diamond shape. So does anyone know what it might be?

Thanks for looking!