The defeat of the rebel armies has left many Irish fighting men without much future in their own country. Large numbers of them leave Ireland to take service as mercenaries in the armies of foreign rulers.
The soldiers in London are probably preparing to travel to Flanders to join the King of Spain's Irish regiment there. Many of them are gentlemen. They have their wives with them.
The English government encourages the recruitment of Irishmen as mercenaries. It is a way of getting rid of able-bodied men who might otherwise make trouble. At this date, three out of four who go overseas never return to Ireland.
Over the next couple of centuries, many Irish soldiers will pass through London on their way to join foreign armies.