8 hour layover Paris tou
 

In planning a trip to Scotland and Ireland next year, we could book the flight so we have at least an 8 hour layover in Paris. Is that time enough to leave CDG airport, hire a cab to drive us to/around the city, have lunch and return in time to make the next flight? Would we need to go through customs coming and going? Would we need to pick up our checked bags? Any other issues?

Any time you land in another country and leave an airport you have to go through immigration (it's not Customs - that's when you have something to declare). Unless your bags are checked through to Scotland/Ireland, yes, you'll need to deal with them. Yes, 8 hours is enough time for a look around Paris and a meal. Just be aware that you should plan to be back at CDG at least a couple of hours before your next flight.

I don't think it's worth the trouble and cost to do that. I would never do that in any major city just because I were passing through, so I would not book the flight with that long a layover to do that. Do you realize how much that would cost you (hiring a cab to drive you around the city for a couple hours?).

I could see doing that in a city that was manageable and where I could use public transporation easily to/from the airport and didn't have to deal with luggage. Like Barcelona or Prague, or even Madrid.

I would strongly advise against it, particularly after an overnight flight.

On arrival in Paris, it's about an hour from gate to baggage collection, another hour to/from Paris/CDG each way, and you need to be back at CDG at least two hours ahead of your departure to Scotland/Ireland. That's five hours, leaving you, theoretically, three hours in Paris. But, know that there are any number of contingencies. Traffic renders the RER more efficient/reliable than a taxi for airport transfers. You'll need to store your bags, if you're flying on two tickets, for which you'd need to allow at least another hour.

Doing this after an overnight flight, even if you've managed some sleep on the plane, is particularly risky. You never know when jet lag will hit suddenly and your legs turn to noodles, which could subject you to costly errors, injuries, etc.

In my experience 8 hours may be just enough time to make your connection at CDG.