Well, the fresh eyes of the other researcher seem to have worked wonders. She has checked her records more fully, and whilst she did not have a Henry Hugh listed, she did have a query that there may have been a 'Harry' in right timescale. Anyhow, spurred on by my info about Henry Hugh, she had a good look on the 1911, and found:
http://www.census.nationalarchives.ie/pages/1911/Waterford/Ballyhane/Kilderriheen/666945/Apart from a few minor discrepancies (least of which being that Rebecca states 2 born/2 living), it certainly seems to fit as being the missing couple. So, they certainly did move around a lot, as this is now them in Waterford.
I have checked the births of the two children and found index entries for each of them in Tullamore - so looks like they returned to Rebecca's home area (and area they married), had the next two children, then headed to Waterford.
A quick check of Kilderriheen reveals only 3 properties. The McKinley's are at property 2, whilst at 1 is, I would guess, his employer:
http://www.census.nationalarchives.ie/pages/1911/Waterford/Ballyhane/Kilderriheen/666944/(could the Sir John Keane at above census be
http://en.wikipedia.org/wiki/Sir_John_Keane,_5th_Baronet mmmm....if so, I wonder if there would be estate papers somewhere detailing his servants etc??)
which again makes me believe we have right couple as this is similar type of employment Henry had at 1901 and in what looks to be a similar type of household.
Mind you, dear knows what become of them thereafter.
edit:
it does look to be same Sir John Keane, as his daughter and son of same names as those on the census return are also noted at:
http://thepeerage.com/p5473