How exciting this query is turning out.
In case its of use ahead The Ros Davies website had two clues quickly found and there maybe others ie
Lieut. John Henry DAVIES R.M. . Donaghadee of Shore St, Donaghadee;harbour master in 1846 source Post Office Directory
Lieut. J.H. DAVIS . Donaghadee chief officer of coast guards & harbour master in 1843 source Post Office Directory
This also shows why Ros sometimes combines two surnames in the same pages. She has noticed an overlap of surname spelling variants. So Was the first right at the time as Davis, or did he change or correct his name becoming Davies by 1846.. I would write to Ros when you have gathered your info together .
Another of my searches of the past produced Coast Guards in Ireland , unlike lighthouses and lightships coming under one umbrella ie The Commissioners of Irish Lights, 16 Lower Pembroke Street, Dublin 2. With publications eg Beam: The Journal of the Irish Lighthouse Service (periodical publication, now annual). Before I leave the irish lights there are good articles about the Royal National Lifeboat Institution and crew members search for Donaghadee in source
http://www.rnli.org.uk/rnli_near_you/ireland I continued a hunt for Coastguards and suddenly found COASTGUARDS OF YESTERYEAR. Search for Donaghadee in source
www.coastguardsofyesteryear.org. it is useful to remember the two current Governments , the british and the irish adminster many things around the coastline.
There are books about Donaghadee as well.
Yep, it is a great site, Good further luck, Jim