Can a driver overtake immediately before the brow of a hill in Northern Ireland?

No. The Highway Code says not to overtake when you cannot see far enough ahead to know that it is safe.

Rule 166 specifically identifies the brow of a hill, hump bridges and bends as examples of locations where visibility may be inadequate. Even with a broken centre line, do not assume that overtaking is safe. Wait for a clear view and enough road to complete the manoeuvre without forcing another road user to react.