The ancient site of execution in Derby is at Nuns Green, which is on the northwest outskirts of Derby on Friargate. A special gallows however, was erected for the Catholic martyrs on the west bank of the River Derwent at St Mary's bridge. The ancient bridge chapel still stands, although the old bridge was rebuilt in 1794; parts of the old bridge are still visible. (Grid ref: SK354367)
The execution most likely took place at the west end of the bridge on the open green area rather than on the bridge itself. Here on 24th July 1588 three priests were hung, drawn and quartered for their priesthood. All three were beatified by Pope John Paul II in 1987:
Blessed Nicholas Garlick.
Blessed Robert Ludlam.
Blessed Richard Simpson.
Their par-boiled quarters were arrayed round the entrance to the bridge chapel. That entrance no longer exists as Bridge House was built onto the west end of the old chapel. A memorial to these great priests is set into the old stones of the chapel's north wall - photo below. In the third photo you can see the tower of the lovely Catholic church of St Mary which was built nearby.
If you are visiting Derby, or passing by, this is a very special place to visit.