It looks quite likely that this is one family group, the three baptisms at St David's in the early 1850s, the 1851 census in David Street, and the deaths and burials of Johanna McCarthy (nee Sullivan) in 1855, and 2 of her children, John aged 10 days in Feb 1850 (which then agrees with the 1851 census), and Mary in 1860.
But, i still can't link them to the James McCarthy who married Ellen Barry in Cardiff in 1880 and were then living with James' father in 1881, as i can't find the Jameses in 1861 or 1871. There is one good looking family, with a new wife / mother called Margaret. But james and margaret are on the 1881 so this cannot be the ones i'm looking for.
And i can find no record of a Lewis Sullivan (who registered Johanna's death in 1856) in Cardiff at all.
Any help would be much appreciated. Thanks.