Finding an old friend

The gut may have a GP up there, you could ring around a few of them and see if he is on their books, the other option is to go to his last known area and do as somebody else suggested ask aound pubs / post offices. You could also try the local postman in an area.
If he owns property you could do a title search.
Has he an ESB connection in his name or other utility, you might get a sympathetic employee that would drop him a line for you if he has an account.

The other option is try the local gardai in case they have come accreoo him.
best of luck

There is no way a GP will give out patient information.

As he was/is a tenant, it is unlikely that he will own a property.