I had my website on mydomain.com for a few years and was number 1 on Google for my market. Due to a bad registrar and my own lazynes i lost my .com a year ago and it has taken me nearly a year to get it back. In that time i have put my site on a local domain mydomain.ie and changed a lot of the back links to the .ie domain so it is showing up in the results but not as well as my .com did. My question is which one do i use now and which one do i 301? Would i be better to switch back to the old domain and would it still carry the same value with Google?
After a year, very doubtful indeed. Google has probably given up, especially if you changed the a lot of the back links. My suggestion would be to roll with the new address now, and continue improving that. Unless there is some decent reason to change, and you know what you're doing getting your redirects, back links, etc, etc sorted for another change, then I can't see the benefit. And after one year without using the old domain, I doubt there will be a really strong reason, bar maybe some old visitors trying the URL on a whim.
Definitely set up new redirects, etc, on the original domain to point to your new one, making sure you catch any authority or link juice you might possibly still have.
Agree with Leonard Challis. If you do decide to move things over again (and I wouldn't without a good reason to) put together a migration plan. Tips below:
Good Luck =)