Déan an fál nó iocfaidh tú foghail.

Make the fence or you will pay the plundering.

Note: Robert Frost quoted an old English proverb similar to this week's seanfhocal:

...
He is all pine and I am apple orchard.
My apple trees will never get across
And eat the cones under his pines, I tell him.
He only says, 'Good fences make good neighbors'.

Frost lived in New Hampshire where the fences are like those of Ireland, made over a long period of time from rocks without mortar. The rocks were extracted from a stony soil by persistent farmers. Frost goes on to wonder:

...
'Why do they make good neighbors? Isn't it
Where there are cows?
But here there are no cows.

But there are cows in Ireland. Is it fences that will prevent plunder? Read The Táin and discover that no fence can protect a cow from the Celt. There is another meaning of "foghail", trespass. Considering the current politics of Northern Ireland, Frost was correct when he concluded that:

Before I built a wall I'd ask to know
What I was walling in or walling out,
And to whom I was like to give offence.
Something there is that doesn't love a wall,
That wants it down...
"Mending Fences," from North of Boston, 1914.