If not for collins, why is it called the rebel county?

In what is surely a very Corkonian twist, the city was judged to be rebellious because of its loyalty to (one version of) the British Crown and the dispossessed House of York in the English War of the Roses.

When a young and handsome Flemish man called Perkin Warbeck arrived in Cork in 1491, he was proclaimed to be one of the princes in the Tower, the two sons of King Edward IV who had mysteriously disappeared after they were locked up in the Tower of London by Richard III. The merchants and burghers of Cork supported Warbeck (or Richard, Duke of York as he claimed to be) in what became a futile effort to reclaim the throne for the Yorkists.

And Cork was thus declared a Rebel county by King Henry VII (who later pardoned Warbeck's Irish supporters, remarking: "I suppose they will crown an ape, next.")