To make the half double crochet stitch (U.S. abbreviation is hdc): It has a single wrap just like the double crochet. The needle is inserted into the stitch, I yarn over and then pull through the stitch. Now I have 3 threads on my hook. I yarn over once again and pull it through all three stands on the hook.
I use a lot of half double crochet 2 together (abbreviation is hdc2tog) in my patterns also. This is just two half double crochets in the same stitch. Making one hdc, I don't pull through the three stands but instead make another single wrap and insert it in the same stitch. I yarn over, pull through the stitch, yarn over and pull through all five strands on my hook.
I am currently working on a design that uses some Irish crochet techniques. One of these is using 3 half double crochet cluster stitches (a form of a clones knot) which is worked in my pattern around the chain instead of in a specific stitch. The 3 half double crochet cluster (U.S. abbreviation 3 hdcCL) just adding a third hdc to the stitch, yarning over and pulling through all the threads at once.
It is closed with a slip stitch in the top. This made a pretty little knot on the spurs of the Columbine flower I am designing
Finally, I needed a little larger knot on some stamen for the Columbine flower. For this I used a 4 hdcCL stitch. Again, I made 4 unfinished half double crochet around the chain (instead of in a specific stitch), yarn over and pull through all the strands on the hook.
Again it is closed with a simple slip stitch which makes a wonderful little Clones type knot on the end of the stamen.The final stage to finish the stamen and spurs of the Columbine Flower design is to slip st around the chain that the cluster stitch is made on.
Now I use the chain as a padding cord. Instead of single crocheting in each stitch I single crochet around the chain the number of times specified in the pattern.
I love Irish crochet and have found that many of the techniques work really well with larger thread and yarn and comes out beautifully. The Columbine flower is so beautiful and I just had to see if I could make something in crochet that would look close to the original flower.
I hope this tutorial is helpful and that it encourages others to not fear using Irish crochet techniques. It really is just crochet with some embellishments.