Mary/Sue.
Thank you for your thoughts.
Unfortunately I believe that both William Mcmurchie's died soon after they were born so I have had to discount them.
I must admit I had not considered the fact that William might have stayed with the Muldoon side of the family - unfortunately as the 1881 Irish census records have been destroyed it will be difficult to prove.
I have requested the film from the LDS showing the christening records, so I am hoping there may be more information on that.