yes, it looks as a simple vocalic fluctuation,
see Thurneysen GOI § 104: árusc, árasc < *ad-rosc
>>arusc ré(i)re - "a saying (word) of will; for instance; Lat. exempli gratia"
>>(DIL sub ríar)
>Exactly. :-) Since DIL gives "árosc" with a
>long 'á', I suppose the most correct version
>ought to be "árusc réire". Can the "-usc" in
>place of "-osc" be attributed to the use of the
>word as a prepositionless dative, or is this
>more likely just a spelling variation?