Thanks Jan & Annie for your replies, I'll follow the leads.
The details I have on my gt.granda are:
John Francis MADDEN born Cork - son of Owen MADDEN (constable in RIC until about 1898) & Mary RING. He married Ida Rebecca BEATTIE in May 1898 - his address was Divis St. In the 1901 census he had 1 child (Helena) & lived in Nile St. My Granny was born in 1903 at which time they lived in Spruce St. His last child was born in 1908 (but died 1910 & was buried by his mother, so I assume his dad JFMadden was not around). His wife remarried in 1914. That's all the info I have, my mum remembers being told that he died in the war, but doesn't know anything at all. I'm guessing he would have been a reserve, so possibly called up to the Royal Irish (rifles or fusiliers).
I'll have a look at the links you suggested, and any other pointers you can offer would be welcome.

Thanks again.
JoMalube