Shooter's Family
Probably Sean's father, who is absolutely certain that his son couldn't have done something like this -- not only was he always the gentle soul in the family, he was a fairly terrible shot. Said father might well have some IRA ties in his past (probably entirely red herrings), which the police should catch onto before terribly long and get suspicious about.
Possibly also an estranged sibling, who is bitter about the whole family and more ambivalent about whether Sean could have done it. Likely actually understood Sean's idealism, and always thought it would get him into trouble. This might be part of how the larger plots come into view, with someone who knows that he would only do something like this for good reason.
