Struggling to research irish ancestors

Hi thank you for reading

I am really struggling to research my Irish ancestors from Waterford. i don't know why but i suck at it!

I am English and have a much easier time doing my English side.

I have a subscription to roots Ireland but i am just struggling.

My great-grandmother Margaret had eight siblings, all of whom i am struggling to research, bar one who has a descendent also interested in genealogy. 

Margaret's siblings were all born 1900-1919, and all long dead. so why is it so hard to trace them?

Furthermore, i cannot get back further than Margaret's grandparents.

I know that her father was Patrick Mulcahy, born to Mary Walsh and John Mulcahy in Knockalahara on 13 Feb 1871. 

Mary was married to John on the 14 Feb 1863 at St. Mary's in Dungarvan, Waterford. It was noted that she was from Ballyduff, and their witnesses were John Murphy and Brigid Walsh.

Brigid I am guessing is either her mother or her sister. But I cannot find anything on her. Not even a date of birth!

Mary must have died at some point between Patrick's birth and 19 Feb 1875 as that is when John remarried to Anna Cantwell (Calling himself a bacheoler) and had Patrick's half-sibling, Margaret Mulcahy in 1876.

I know that Margaret's mother, Catherine Ahearn, was born 17 Jan 1876 in Ballingowan East to Maurice Ahearn and Margaret Foley. she had 3 siblings, Mary, and Margaret, born in 1873 and 1874 respectively. Maurice had a brother called David who never married. 

Maurice and Margaret married 2 Jul 1872 in Clashmore, Youghal, and both their father was named John. But that is all i have been able to find.

Are there any Irish ancestry websites that are recommended?