Hi Craft, if all the children were born in Ireland on the 1861 (though I would still look at the GRO index to verify the younger ones definately were) then the family came over together, if ironmonger looks at the other Irish families in that Parish (if it is the right family) to see if there are any clues there, but even if the heads of house had given their birthplace, the enumerator was only required to give the Country of Birth if outside England or Wales.
In my book, Irish Genealogy, A Record Finder, it prints the geographical analysis of civil registration births for 1890 in all 4 provinces, it is a very useful index that can sometimes point you in the right place, as some names really did remain common in particular places.
Grogan entries in 1890 numbered 44 9though 5 were a variant spelling) but when the Province entries are broken down, it is clear that the greatest number of registered births for that surname occurred in Leinster. the enreies then broken down into Counties lists Dublin and Topperary as the Counties with the most entries, The way to use this index in relation to the census survey of other orish familes in the same English District, is to compare the surnames of those census families with the instances of the birth entries of 1890 to find common overlaps.
this is a complex way of narrowing down likely counties for research back into the 1800s. It is time comsuming initially, but much research has gone on in recent years to establish migrations in large numbers organised by parish or large employers in England and Wales, this is why many Industrial or large English towns very quickly developed their own Little Ireland (as many towns called them).