I'm assuming it's not glazed? There's a lot of good research debunking the whole idea of crocks improving drainage anyway.
Ivy doesn't like being waterlogged & all plants need air at the roots so I'd suggest filling it with a nice loose soil mix that crumbles after you've squeezed it in your hand. Plenty of gravel, even just mix the crocks into the soil.
Water well but carefully only after the surface is dry

The gravel really won't help much. The water needs a place to drain so the roots dry out between waterings. The plant doesn't want to be constantly moist. You can simply drill a hole in the bottom with a diamond tipped drill bit & your power drill. I've done this with my Mickey Mouse coffee mug from Disneyland. I obviously didn't want to throw it out (it's genuine), but it was too old to drink from. It's now a flower pot for my African Violet & I can water it from the bottom. You can do it with an ordinary drill bit but this will take a long long long time and it will get super hot hot hot. It can snap on you (like mine did).

The diamond tip is more expensive but it works like a dream and you can turn anything into flower pot - old tea pot, salad bowl, the dog dish, lamp shade, etc. You can get real crafty with it.

I agree - if it rains hard the water would still collect around the crocks and keep the compost too wet.

It would be a good idea to find out why the ivies are not thriving where they are before you move them- ivy is usually a very tolerant plant. Maybe a photo would help us to help you as its no good just transferring the problem to another pot.You can add a photo in the same way you asked the question - you'll see the option to add one on the question form. Info about how long they have been in the container and where its sited would be useful. I know this isn't what you asked but it might help to prevent the same thing happening again.

Hi BG, I thought that even without a drainage hole a water/air balance could be made by incorporating coarse materials into the soil? Assuming that the pot breathes & loses water okay...
Stera has a good point though, are the plants not doing well in the current container because they're not being treated well?

If you are going to drill a hole in a ceramic/clay pot, first wet the spot where you are going to drill. I use a masonry type bit and slowly start. I kept wetting it and drilling slowly. I have been able to do several pots without breaking or cracking them. I used a ¼" bit and drilled several holes in the bottom.

Good point and don't apply too much pressure. Let the drill do the work.

The other thing you can do is use sticky tape (not sellotape/Scotch tape, too shiny and slippy) - I use masking tape to cover the area I want to drill, then drill through that.Stops the drill bit slipping and reduces the risk of cracking in the pot.
Fact is, as others say, your pot has to have at least one drainage hole for any plants you want to grow - using gravel or stones in the bottom just encourages a perched water table, doesn't work.

Would putting a pot inside the crock on a support to aid drainage not work?

Not really Darren...it will probably just sit in a reservoir of water. I’ve tried many times and it never works. In fact good drainage is probably the most important thing for containers. You’ll need to get your drill out! I’ve bought ‘self-watering’ pots in the past and they are basically as you describe....a pot in a pot. They can be dry on top but sitting in stale stinky water at the roots. Terrible things!