Daithi,
I had checked the O'Connell Street details - but can't quite see how the Wm. Kertland mentioned in the notes squares with the Wm. Kertland who lived in Preston Street. I suppose that I'm asking: are there two Wm. Kertlands in the equation? On the other hand, according to another reference that you supplied, Joseph Kertland was the son of Wm. and Elinor. Perhaps there was simply a change of address...
Overall, I'm still not sure that the chemist was the theatre man too...In fact, I'm getting slightly confused (not unknown) in that the whole chemist line of enquiry may be a distraction, partly of my own making.
I don't think that the later references that you've posted help in establishing 'my' Wm. Kertland's own domicile; tho' the links (Greham - sic) are interesting.
roly