Unless there is a very good reason not to, why not have him castrated, then your options are greater. You could then either have another male, or a female which you could then eventually have spayed. I think he would love a bitch puppy to play with, he is still very young himself and the two would play together. If you have him castrated before getting the bitch and then wait a little while for it to take effect, he will have no issues with having a female on his territory. Also I would say an Irish is a much different proposition to a Ridgeback male, and even an entire one would be far less likely to have dominance issues even with another male.