Catherine
I have been looking at below, I can see have been researching for some time.
State registration:
https://www.irelandxo.com/ireland-xo/history-and-genealogy/ancestor-dat…
I then found this posting. For Mgt Hood Clarke's marriage the certificate indicates home address Drumhalla
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1852/09435/5418151.pdf
(The Belfast Newsletter extract)
[Married] MILLIGAN & CLARKE At Kilmacrenan, on Monday, the 20th inst., by the Rev. Henry C. Cochran, Mr. H. Milligan, of Killygarvan, to Margaret Hood Clarke, fifth daughter of the late Mr. John Clarke, of Ballymacreely, county Down.
From reading this my thoughts are that as she is not local it is possible that Margaret held a position in Drumhalla House, if search online will see that it is now a wedding venue. I can see that when submitted a previous query you were advised that the name Hugh is not in the tithe applotments of 1834, though at marriage occupation is a farmer. I do not know if there is an omission or if he moved to this area and held land for a short time. I searched for Milligan variants in the locality. I also cannot see records for the marriage or births of the children from his first marriage, in Rathmullan-Killygarvan the churches do not have any early records. In regards to naming my impression would be that they picked the 2nd names of their children as a form of respect. The Belfast newspaper announcement is usually indicative of some standing. Possibly problems arose before the departure in 1858, If I can find any more details I will update you.
Have you completed any DNA tests with genealogy companies? while the in common would be low it could provide some more information on the family.
Best Regards,
Marion Whelan - IrelandXO - Ireland Reaching Out Volunteer.