I did mean the Irish
It will happen years after his departure (Johnson has been a useful idiot for the taxdodgers, for ramming some sort of Brexit through by force of personality this last year) but he should forever be tarred as the PM that broke the Union - the hard border down the Irish sea, which his 'deal' negotiated, will see NI inevitably drawn towards Dublin now. Sectarian issues notwihstanding, economic pressure will force the issue.
He's also near-singlehandedly taken a wrecking ball to any hopes of avoiding Scottish independence via both Brexit and his obvious contempt for the Scottish parliament. Support is now consistent for it and an SNP majority next year will be hard to argue against.
Wales isn't lost yet but once the other two start to move away, will the Welsh want a future as England's sidecar? Apparently there's been an uptick in support for independence there (though it's still in the minority). Could that accelerate in the decade to come?
Just like every other job role he's failed at, we'll soon be able to toss 'Minister for the Union' onto the flaming yet massively profitable wreckage of Johnson's 'career'.