He could sell a priest a pig (and the parish clerk a piglet).
Note: This is said of someone with great powers of persuasion. He could sell ice to Eskimos. He could sell sand to Arabs. He could could even convince the most learned person in the parish, the priest, to buy a pig. A priest needs a pig like an Eskimo needs ice, like an Arab needs sand.
Note also: The Irish word ‘ar’ is a simple preposition. A preposition combines with a noun or pronoun to form a phrase, e.g., ‘ar shagart’ (literally: on a priest). A prepositional phrase can modify the meaning of a noun, e.g., ‘muc ar shagart’ (literally: a pig on a priest).
A preposition can also modify the meaning of a verb. In this case, the verb ‘áitigh’ means ‘occupy, settle down to,’ or ‘argue.’ However, the preposition ‘ar’ modifies the verb ‘áitigh’ to mean persuade or convince.
This week’s proverb literally means, “he could argue a pig on a priest.” That is, he could get the priest to take the argument on him, i.e., he could ‘sell’ the priest. This is ‘sell’ in the sense of persuade. ‘Sell’ in the sense of transact’ is ‘díol.’ Dhíolfadh mé muc leis, má tá aon airgead aige. (I would sell him a pig, if he has any money.) ‘Díol’ also means ‘sell out = betray’ or ‘pay.’