RC records for the parish of Mellifont go back to 1821, and are available in the National Library on microfilm : Pos. 5599 and in the PRONI in reference MIC.1D/53
For civil records I think the following are the best matches :
Name: William James Campbell
Registration district: Drogheda
Event type: Birth
Quarter and year: Apr-Jun 1891
Volume : 2 / Page: 432
Name: Mary Margaret Kiernan
Registration district: Drogheda
Event type: Birth
Quarter and year: Jul-Sep 1891
Volume : 2 / Page: 394
and a possible match for the marriage with a different spelling of Mary's surname :
Name: Mary Kierans
Registration district: Drogheda
Event type: Marriage
Quarter and year: Jul-Sep 1908
Volume: 2 / Page : 449
Name: William Campbell
[same references as Mary]
The civil certs should show a little more details - e.g. occupations, and maybe slightly more detailed addresses if the events took place in a town - for rural areas address is usually given as the townland name.
For notes on ordering certs, and the details they contain see :
Ordering Certs from GRO Roscommon Details included on a Marriage Cert Details included on a Birth CertI dont see a townland named 'lugboiy' maybe Lurganboy mistranscribed ? This seems to be reasonably close to Drogheda as it's in the same registration district
Shane