Hi James -- I would definitely only leave him inside during the day -- in general an outdoor run would not be appropriate for a cavalier anyway as they are an indoor housedog and toy breed, less a dog that is happy alone outside. Also you are right, there is a worrying rate of dog theft in Ireland and cavaliers are one of the favourite targets. Many are stolen to order and sold on to people who think they are getting a rescue dog or worse -- to puppy farmns to spend the rest of their lives in a small cage, used for breeding.
Some are --horrificly -- used to bait and blood fighting dogs too.
If you are coming home at lunch for a walk/run you are already doing a major positive step for good care for a dog that otherwise is alone during the day. You might give him a proper short walk then and make sure he has relieved himself.
Here are some more ideas:
Can you put him in one room -- a utility room, the kitchen, a spare room -- and leave a radio on (my dogs like radio 1 or Lyric FM
(classical music is known to be very soothing for animals). He also needs fresh water available, a bed, and anything risky he could chew on needs to be removed (eg cords or personal items like shoes).
I recommend getting him a Kong toy which you can stuff with some kibble and say, chicken paste, and freeze. You can get smalls or mediums for cavaliers -- but be sure to subtract this food from his daily ration! Then give him the kong when you leave. It will keep him busy for ages, working to get the goodies out! Also some other safe toys should be left with him too. You can also hide an item like a kong or his favourite toys, and leave him to find it while you are out. You could give a second kong when you get home at lunch, for afternoon play. Note you could used chopped bits of carrot or green pepper surrounded by the paste as well to lower the calories in a kong. Most petshops will sell kongs.
I'm sure others will have ideas as well as other things that can be put in a Kong. You can read more about kongs here:
http://www.kongcompany.com/worlds_best.html
I wouldn;t worry about the time he is on his own as he is getting out for that all important midday break with you and relieving himself and getting a stretch. What you want to do now is make sure he is safe inside and has some things to keep life interesting while indoors.
Also: I need you to try and bring down the size of your avatar. There's maximum dimensions and info on how to do this in the Getting Started section. The max size -- to keep the board from getting stretched way out to the side -- is the size of my own. I can help you if you need some help getting the sizing right.