I personally hate to start crocheting in rows with a chain base, especially when i have to crochet in the two sides of the chain (like in the case of irish leaves), and i found this tecnique to be very useful to give speed to my projects...so, let's start!
Crochet 2 chains, insert the hook only into the back loop of the first chain...
... yarn over, pull out a loop, yarn over and pull out throught the two loops.
We have crocheted a single crochet :)
To continue the double chain, we have to insert the hook...well, i don't know if there is a specific name...where you see the arrow :)
Then yarn over, pull out a loop, yarn over and pull out throught the 2 loops.
Repeat this step till the desired lenght, you can count the chains on the right.
Once achieved the desired lenght, crochet one/two/three chains depending on the stitch you'll go to use, and work on the left side of the double chain, in the holes you see near the chain.
You now can picot and work in the other side inserting the hook directly in the holes, or you can turn your work and leave a chained border easy to be crocheted or to be left as it is. :)