Thanks for your answer - I'm assuming the device you are linking to is the "Actiontec Ethernet to Coax Adapter Kit for Homes with Cable TV Service (ECB2500CK01)".
All the reviews for this adaptor are great - I'm just a little in the dark as to how I would use it for my application.
For starters, does the modem have to be in the circuit somewhere? The coaxial cable coming from the house to my workshop does not go through the modem or the wireless router (it is split off from the service as soon as it comes into the house).
Also, will I need two of them, or only one? I'm not seeing how they would have to be set up to work for me.
I have not used these but it seems that you would need to attach a network cable and the coax cable at the transmitter end (your house) and the receiver end in the outbuilding to split them apart again.