If most Protestants were not 'informers' that would probably be because they had no information to pass on. However I think Sarah unwittingly and superbly illustrates the reason why using the word 'informer' in this article is utterly inappropriate and far from neutral. 
She says "common usage in Ireland [so] describes (then and now), local inhabitants who give information to the British." 
But Ireland then and now also includes the Protestant north whose Protestant inhabitants see themselves as British and would no more think of not 'informing' than would an American who spotted an Al Quaeda operative setting up an ambush in Chicago. And they would not be pleased to have the epithet 'informer' thrown at them. 
Irish Protestants in the south then, or the north now would never use the word 'informer' in this context and their view should not be disregarded in Wikipedia. Sarah may assume they do not exist or are irrelevant but their continued existence is the reason she is editing this article in the first place. Neutrality requires another phrasing.