Chomh glic le sionnach.

As clever as a fox.

Note: "Sionnach" is one Irish word for "fox." Ulster Irish, like Scots Gaelic would also use another word for "fox," namely "madadh ruadh." "Cho charach ris a' mhadadh-ruadh." (As wily as a fox)-- Scots Gaelic. This expression would be spelled "madra rua" in the official standard Irish, and literally means "red dog."

Seán Ó Dálaigh wrote a short story called "Dioltas an Mhada Rua," showing that a fox is not only clever, but also vengeful:

Díoltas an Mhada Rua
Seanfhocal is ea -- chomh glic le mada rua. Agus i dteannta é a bheith glic bíonn sé díoltasach. Thaispeáin sé d'fhear ó Dhún Chaoin go raibh sé díoltasach mar b'air féin a d'imir sé an díoltas.

Revenge of the Fox
'Tis a proverb -- as clever as a fox. And when he is in a fix he can be vengeful. He showed a man from Dhún Chaoin that he could wreck vengeance when he must be avenged.

Seán Ó Dálaigh lived in the Kerry Gaeltacht. Hence, he uses another dialectical spelling of fox,"mada rua," quoted above in the title and the first sentence of his story. Ó Dálaigh is the "Máistir Ó Dálaigh" that Peg Sayers writes about in her autobiography.