From Paris, you can use RER A or RER C. The time it takes to is about 1 hour. You might consider the bus for faster travel because the metro has more than 20 stops from Paris to Eragny.
In Eragny you have to use the bus. Taxis usually don't want to go to short distances like 3 km. There are not walkways on every road, so walking becomes a hassle. Study the bus routes. Routes and time tables are displayed on bus stops. Some buses stop as early as 22:00. For Cergy Pontoise, see interactive map. Your metro tickets are also valid on the bus (provided that you bought them for Zone 5).