This may not truly be considered a Celtic heart, but it is my attempt at one and I decided to give it that name. It's more of a tatted heart with a Celtic trinity knot in the middle. This is a really simple pattern that doesn't take very long to tat.

Step 1: Materials

Supplies

Red Tatting Thread
Tatting Needle
Paper Clip
Jump Ring
Chain with Clasp/Necklace
Mod Podge
Sponge Brush
Iron

Step 2: Inner Trinity

To tat the center you are going to have to work with many double knots on your needle at once. You can do it in sections, but I don't like the look of it when I've done it that way.

Tat 60. Put the paper clip at the end and pull the knots off without letting the end close. Weave the strand (Pictures 2-6). Close.
Here I straightened out the trinity knot so that the knot is no longer in the center of a tip of the trinity knot and is in center of a side that overlaps with another side. (Picture 7)
Thread through the other part so that it stays together. Knot. RW. (Picture 8 - 10)
R 8 cl knot. (Picture 11)

Step 3: Outer Heart

To tat the outer heart, you can reverse work, but I just worked whichever way the threads would let me go.

Tat 30 knot.

R 8 cl. k.

Tat 30 connect to where you started the first 30 at the top. Double knot and hide ends. Make sure this strand of 30 is the same length of the first strand of 30 or the heart will be lopsided.

Step 4: Iron and Mod Podge

Iron your heart and try to get it to lay the way you are going to want it to be (heart shaped). You can try to spray starch it, but I wanted to make sure it would stay so I decided to try mod podge. Carefully spread a little on your heart with your sponge brush. If you have too much anywhere, dab it off with a paper towel. Do this to both sides.

Lay to dry and check on it to make sure it doesn't stick to whatever you lay it on. Make sure it isn't sticky at all before you attach it to your necklace.

Step 5: Make It a Necklace

Attach a jump ring to the top loop we made on top of the trinity knot. Use the jump ring to attach to the necklace and you are done! If you have a specific front and back, I did, keep that in mind when connecting it to the necklace.

Enjoy.