What would make a French invasion of Britain viable?

How could a French invasion of Britain during the Napoleonic era be made to work?

Gain sea control long enough to bring a substantial force across, and avoid depending on an Irish rising.

A workable invasion first needs control of the sea long enough to bring a substantial force across. Its strength should not depend on the promised arrival of an Irish army. These are preliminary conditions for a viable operation; the brief answer supplies no detailed campaign plan.