Looks like the often misnamed 'Heat Lightning', which actually is 'cloud - to - cloud' lightning as opposed to 'cloud - to - ground lightning' . The misnomer 'heat lightning' stems from the fact that often you cannot hear the clap of thunder in such events. Living in New England , I've been through a few severe snowstorms with lightning (thunder snow) present, and wow, these are huge storms with cold air on the north and west side of the Low Pressure area, and warm moist air, on the south and southeast side, feeding in from the southern states and from the southeast warmer waters of the Atlantic Ocean. A marvel of nature to behold, but a pain in the back from snow shoveling the next day (I won't tell you where the real pain is

).
Edit: I didn't realize how old the original post was! Say, Chris, are you from Ireland, are ya?