Numbers 22:36

unto the City of Moab unto Ir of Moab. The Heb. form of the name Ar of Moab (Numbers 21:28).

which is on the border of Arnon i.e. the border or boundary formed by the Arnon. Moab at this time possessed no land north of the Arnon; Ir (Ar) of Moab thus lay on their northern border. The following clause defines the locality further.

which is at the end of the border Since Balaam was coming from the east, -the end" must be the eastern end; that is to say Ir (Ar) of Moab lay somewhere on the upper, eastern, course of the Arnon.