Thanks so much for these replies!
I hadn't seen that 1871 entry at all!

I have found these two possible marriages of a James WALMSLEY and Margaret ? in Ireland on the LDS website. The ages roughly stack up.
a) James WALMSLEY & Margaret THOMSON - 15 NOV 1848 - Killesher, Fermanagh, Ireland
b) James WALMSLY & Margaret HARRIS - 01 DEC 1849 662, Desertserges, Cork, Ireland. Husband Age at Marriage: 36; Wife Age at Marriage: 30
Would be nice to know where they originally came from. Will try sourcing the births of the various children for clues...
Alice's middle name was WATKINS and her James's was FITYZALLEN, so I wonder if these names could be clues to a previous generation?!
I also believe that James was a Custom House Officer at Holyhead (received information, no real source) - would there be any way to trace him through the coast guard/custom office ?
Thanks so much for your help! Really appreciated as I'm very new to all this really...