If you believe everything you read on a death record--which I dont-- then Daniel Shannon and Catherine (Shannon) McClay were not brother and sister. Here is Daniel's death record information:
Daniel Shannon, died 7 Nov 1901, age 60, at 256 Hancock St, Bangor, widowed.
father: John Shannon, a farmer
mother: not known https://familysearch.org/pal:/MM9.1.1/KCD2-7X9
Remember: this man died widowed and on his own, so any information about his parents' names is really suspect.
in 1900 he was at that address in Bangor:
at 256 Hancock St, own household
Daniel Shannon, Aug 1841, widowed, saloon, nat. 1863 https://familysearch.org/pal:/MM9.1.1/MMLR-W9K
I couldn't tell if you were asking if these next two men were relatives or not, but they are. Here are two more death records:
1. for the son of Daniel Shannon and Annie ___:
Daniel P (abt 1880 - 6 Jul 1911, Bangor)
died age 31, single, a clerk, father: Daniel, a laborer
died of TB
both parents born in Ireland http://search.ancestry.com/cgi-bin/sse.dll?rank=1&new=1&...
2. for the son of Henry McClay and Catherine Shannon:
Daniel H McClay
died 16 Sep 1919, age 53, married, residence: State St.
parents: Daniel McClay, born in Ireland, and Catherine Shannon, born in Bangor
cause of death: internal injuries fractured sckull, shock, due to accident. fell from runaway team horse dragging him for a time then throwing him and jumping on him http://search.ancestry.com/cgi-bin/sse.dll?db=MaineDeathRe&a...
Note his death record said his mother Catherine was born in Bangor, but her 1870 Census -- which she probably provided the information for -- said she was born in Ireland. That's what I mean about death records.
The Allen-McClay Tree at ancestry.com says
Catherine Shannon, wife of Henry McClay was born 1833 in Ireland, died 25 Apr 1875 in Bangor. Daughter of Hugh Shannon and Sarah A Gillan http://trees.ancestry.com/tree/32394111/person/18248909374
the tree was posted by a gr gr grandson of Henry McClay and Catherine Shannon
In 1870 your Daniel, husband of Annie, said he was a citizen. Also in 1900 he said he was naturalized in 1860.
So here is probably another person you can look at:
Daniel Shannon, naturalized in Bangor in 1876 https://familysearch.org/pal:/MM9.1.1/VXR8-SPJ
There is also a Hugh Shannon in Kingman, Penobscot Co., Maine, in 1880, who was born about 1830 in Ireland. https://familysearch.org/pal:/MM9.1.1/MFQ9-QBJ
On the original, he is listed after a George Shannon, born in New Brunswick, and Hugh is called George's brother.
Toss them into the mix. Maybe Hugh was named after his father and they are all brothers and sister.