A thread on the Great War Forum re records of Irish Volunteers led me to do a quick Google and one of the options that came up was a site to do with Portaferry in County Down, where my Great Grandmother Jane Maria McCullen was born. She was the daughter of Catherine Carson from Dublin and Charles Augustus Edward McCullen from County Louth. They had married in Dublin in 1877, with Catherine listed as living at a very interesting address.
Looking at the website re Portaferry, I noticed that there were references to M'Cullens relating to a John David Sumner who was killed in 1915. His surviving maternal uncle is listed as Mr C A E M'Cullen, RN, chief officer of coastguards, retired. Data that matches Charles Augustus Edward McCullen who had retired from the Coastguard as a Chief Officer (Petty Officer) before the Great War. I was aware of Francis McCullen and James McCullen of Portaferry but had not been able to tie them in any way to Charles.