I can see what looks like James Joseph’s birth in the civil indexes. Registered in Magherafelt, Apr – Jun 1878 Volume 1, page 818. The other brother’s birth seems to be registered as William Patrick Donnelly, Magherafelt, Jan – Mar 1880, volume 1, page 788. I’d be inclined to order one or both of these to get the mother’s maiden name. You should then be able to find their marriage in the civil indexes, provided it was post 1864.
You can order photocopies of these certificates from GRO Roscommon for €4 each. Put the place, quarter, year, volume & page number on the application form. Don’t worry about leaving some boxes blank. Provided they have the references, they should find the certs OK.http://www.groireland.ie/
The birth certs should give you Daniel’s precise townland and occupation and you may then be able to locate where he lived using the revaluation records:http://www.proni.gov.uk/index/search_the_archives/val12b.htm
(There are Donnellys in Straw in the 1870s & 1880 but I didn’t see a household headed by Daniel, so possibly he lived in an adjacent townland. The birth certs should help to clarify that, as will his occupation).