It was inevitable that Clay would die, but just when was the question. In prison, out of prison, but it was gonna happen. I think several more major characters will not have happy endings before the series wraps up. It's destiny. Only one person has a chance of surviving the series, and it's Gemma but she'd be a broken woman.
BTW, Ron Perlman has a great interview at EW.com about his demise.
It will be interesting to see if there are repercussions with the Irish back in Northern Ireland, or they will buy the Clay-Galen shootout.