Ancient Celtic Pottery?

I recently purchased this item at an antiques market in PA. It is marked "Celtic" on the bottom along with a museum shelf number. It measures approx. 5" diameter x 5" high. I am new to collecting old pottery, and was wondering if others thought this was Celtic as well or perhaps Native American? Any idea on time period? Thanks so much.

 

The term "Celtic" has a very broad scope, which is not usually used in ID-ing ceramic types. If you could post a shot of the inside to help identify type of manufacture, that would be useful. I'm checking with some UK archeologists to see if anyone recognizes this...stay tuned.

 

Thank you. I appreciate your help. Here are several additional photos as requested.

 

I posted this item on a few other boards, and was told by a couple of people that it looked to be possibly Mississippian shell tempered pottery. Any thoughts on this? Thanks again.