Celtic Casino is regulated by Costa Rica and powered by Visionary casino games. Both Costa Rica and Visionary do not accept citizens from the United States of America.
I sent an email to the Celtic Casino support to get a clear explanation. Here is their answer :
Due to US legislation, we cannot accept real money bets from US players at this time. However, you can create an account and play for fun indefinitely.
Regarding any other country/territory, we do accept players from most countries in the world (except the US and its territories and Costa Rica) to play in our Live Casino games. Nevertheless, we do encourage players to be informed about their local gambling regulations and to make sure that online gambling is allowed in their jurisdiction.
As part of the US gambling law, Celtic Casino preferred to step back from the US Markets. However, some online casinos still continue to accept US citizens. Check our top 3 US Casinos.
Technically, you could play to Celtic Casino if you signed up from abroad. For instance if you are in vacation in Colombia or in Argentina, you could play to Celtic Casino while you drink your Cuba Libre. However, you should not use a US address and an american credit card (from Bank of America for instance).
This is the same problem as "signing-up from abroad". You could use a VPN to play to Celtic Casino, but you would need to have a real address outside of the US (with electricity bill at your name for instance). If you have an aunt abroad, for instance in the Russia, it could work. Otherwise, in case you win a $7,000,000 jackpot, Celtic Casino could block your funds..
We know that Bovada is currently accepting US citizens. It is powered by Top Game software and offers a nice welcome bonus.