Hi Anne, I found the baptism and marriage you mentioned on Roots Ireland, however the parish is Rush (yes that is a place) it is the parish adjoining Donabate but there is an esturary between them so a bit of a distance by road at the time, Portrane is in the parish of Donabate and you can see the old boundaries here as well as the entries but only search by date and scroll https://registers.nli.ie/
Eleanor was baptised on the 25th April 1790, I see no other children to these parents on Dublin or around, records for RC births are earlier here and there is also no marriage I could find, there are a number of marriages including Thomasine in Rush and Lusk (there are side by side and locally you would nearly say them together) There are also a number of marriages of Harmons in Donabate (Portrane) again on top of one another. Unfortunately it does not give a parent name in any record. No usual for the marriages at the time. Also on Eleanor's transcrition it says in the note William Murray.
Thoamsine married on the 1st Oct 1796 to Peter Reilly witnesses were James Harman and Catherine Kavanagh, parish Rush.
Pat Harman married Mary DArcy on the 27th Dec 1795 in Rush witnesses Pat Toole and Michael DArcy (D'Arcy usually)
Catherine Harmon married Arthur Matthews on the 23rd Oct 1793 in Rush, witnesses were Pat Kearns and Anne Harmon
Nicholas Harmon married Sarah Toole in Rush on the 21st June 1793 witnesses were Austin Maley (Malley) and Peter Harman.
Richard Harmon married Mary Reed on the 10th February 1793 in Donabate (Portrane)
Catherine Harmon married Patrick Cain on hte 8th Nov. 1791 in Lusk (Rush) witnesses were Jane Johnson and Frances Donnelly
John Harmon married Bridget Russell witnesses Pat Flemmon and Mary Lamb on 27th Dec 1809 in Rush
Mary Harmon married Thomas Cunningham in Rush on the 8th June 1800 witnesses were Barney Morgan and Michael Nugent
Sarah Harman married Patrick Fagan in Rush on 16th Dec 1814 witnesses were John & Margaret Cunningham
Richard Harmon married Catherine Kennedy in Skerries (again beside Rush) in 1818 on 29th November witnesses Pat Gardiner and Mary Sever
Without a fathers name it is not possible to say if all brothers and sister etc but the names may crop up in your research.
The name is often associated with County Louth about 15 miles north and one other witness I did not list is also much associated with Louth Hodger or Hodgers, there is a very interesting story form the American civil war of a lady with that name who served as a man, this she was a doctor, I will find it if you are interested, there are also a lot of Harmons married in DUblin city during this period and can be seen here under Church records, civil records much later https://www.irishgenealogy.ie/en/ Looked up Harmon in a book of surnames and back in the fullness of time is was a Germanic name apparently but in Ireland for many centuries now, probably about a 1,000 years at least.