Ar is a simple preposition that means on. It puts the following noun into the dative case and generally lenites, when followed by a lenitable consonant.
Deas is a feminine noun that means right (in the sense of 'direction'); also nice.
In the dative case, the ending of deas is slenderised and it becomes deis.
Thus, ar dheis: on the right.
The following related phrases are useful.