Looks like a small tree, possibly a Rowan species. It's seriously close to your fence and brances have possibly gone under and popped up from the fence from the neighbouring garden. It's worth checking if they have a tree on the other side of the fence. If they don't have a tree, it would be advisable to move it further from the fence, or altogether. Dig it out instead of cutting the stems off as it will only continue to grow under the soil. But Rowans are beautiful trees that can easily be maintained and are a great source of berries for birds in the winter
It does look like a sumach and they have a habit of suckering. Is there a tree the other side of that fence?
Yep, looks exactly like my sumac (we call it stags antler) its common for them to send out runners which pop up all over the place, they can be dug up & repotted, try to take as much root as you can as they dont always take, sadly Be careful of the sap it can be an irritant
