A big thanks for your comments.
Seanmac - what a lot of possibilities! Somewhere, I have an old map of Mullingar, so I should be able to see where they all fit. Marcella had 2 brothers, so hopefully their descendants may be amongst those listed in 1901. I haven't researched them yet.
Shanew147 - Yes, that's the one I found (thanks to you) and have ordered the certificate for. I'm very excited! It's been about a week now. How long will it take???
agho - I have actually seen the Parish Records entry for Lawrence and Mary's marriage. Sponsors were given (but apparently not related) and no parents were named.
There is a Lawrence M'Loughlin (b. Kilbeg - about 20miles from Mullingar) listed on the IGI who shares a father with a Denis M'Loughlin. I'm in touch with Denis' descendants, but can't find the link that this Lawrence is my Lawrence. Very frustrating!!!
Thanks again
Penny