Hi Martin,
Dave Mervyn is my name - Daisymershum comes from not being able to pronounce my name when I was a toddler! I'm from Dublin.
My great grandfather was William Mervyn, a brother of your Thomas Mervyn. Their father was William Mervyn (born circa 1827, possibly in England).
William Mervyn senior married Bridget Dunne in Dublin and had, by my estimations, 10 or 11 children. I have records for the following children:
William Mervyn (my great grandfather) married Catherine Power, Dublin. 1862-1901
Thomas Mervyn (your great grandfather) (living in Innisfallen Parade, Dublin at time of marriage) married Margaret Broderick (Prussia Street), Dublin. 1860?-1902. I have records for three of the four children they had - Thomas Joseph Mervyn born 1891 (was he born in Dublin? I had a birth/baptism cert for him from Belfast), Mary Brigid Mervyn (1892) and William Joseph Mervyn (?).
William and Thomas' brothers and sisters were: Margaret, Anne Marie or Maria, John, Christopher, Bridget, Ellen, Josephine (there was no Joseph!) Mary and Catherine (Kate). All were born between 1860 and 1876. I actually think Bridget (Dunne) might have died giving birth to Bridget in 1876.
Most of these settled in Dublin in the 1880s/1890s after the family had spent a fair time in Celbridge, Co. Kildare while William snr worked as a lodge keeper. Shoemaking or bootmaking was the family trade.
William snr and his sons were all bootmakers and worked in various premises in Dublin. Mostly Innisfallen Parade, Berkely Road and the north inner city.
Some of the girls were teachers in Celbridge and elsewhere, Ellen and Kate. The Belfast link is interesting because Kate married a Vallely and moved to Belfast and Thomas and Margaret (Broderick) also moved to Belfast.
William snr, your great great grandfather, died in Dublin in 1891, aged 64. He was living at 13 Innisfallen Parade at the time. He is down as a bootmaker and widower. He is buried in an unmarked grave in Glasnevin cemetery, the cause of death was 'debility'.
Unfortunately, with living in tenements and te poor conditions/spread of diseases, some of William and Thomas' sibilings died at a young age. Maria died a few months before William snr in 1891, aged 19. Ellen died in 1894, aged 34. Both are buried with William snr in Glasnevin.
My own great grandfather, William jnr, died of phthisis in 1901 in Dublin, aged 39. He was a bootmaker at the time in Berkely Road. He had three sons, William (stone carver), Patrick (clerk, Captain in Irish Army) and Michael (electrical engineer, Trade Union President and Irish Citizen Army member from 1916 Easter Rising). Michael was father to my own dad, Michael.
Of what I've looked into regarding the Mervyns who moved north, these are my census links:
http://www.census.nationalarchives.ie/pages/1901/Antrim/Falls/Murdocks_Street/951955/http://www.census.nationalarchives.ie/pages/1901/Antrim/Smithfield/Albert_Street/972482/http://www.census.nationalarchives.ie/pages/1911/Antrim/Falls__Belfast_/Elizabeth_Street/180817/http://www.census.nationalarchives.ie/pages/1911/Antrim/Smithfield/Albert_Street/184986/