The problem with any use of the word 'informer' is that it is pejorative and used as an heinous insult in Irish republican circles. Informers are thus bad people, but usually people who are in some way treasonable. This makes the word's use completely inappropriate in relation to the Dunmanway massacre as none of those killed had any loyalty to the IRA. They may well have been close to the security forces (RIC or army) and helped them but this is what they were brought up to do. 
A neutral (NPOV) phrasing might be that they would probably have assisted the RIC and army or been informants. Their killings however had much more to do with vengeance (for the O'Neill shooting) than a coherent attack on 'informers' as republicans would have seen them.