Tana,
The 1943 street directory lists Agnes McDowell at 33 Royal Avenue.
https://www.lennonwylie.co.uk/alphanames1943Mc.htm
A 1922 advert in the Belfast Newsletter has an advert for Nobletts sweet shop at the address. The advert is for Easter Eggs and chocolate products.
1939 Street directory has Agnes McDowell at 33 with Nobletts next door at 35.
https://www.lennonwylie.co.uk/qrcomplete1939.htm
No sign of McDowells in 1932:
https://www.lennonwylie.co.uk/qrcomplete1932.htm
Belfast Telegraph of 2nd April 1943 has an advert from Adlestones, the jewelers, 46 Ann St & 33 Royal Avenue (offering good prices for silver plate etc).
Belfast Telegraph of 26th September 1963 has an advert from HH McNeilly of 33 Royal Avenue offering engagement and wedding rings for sale.
Belfast Telegraph of 28th March 1925 has an advert for Dunseith & Flack of 33 Royal Ave offering Indigo Blue Serges for sale.
Irish Weekly and Ulster Examiner of 9th July 1938 has an advert for Agnes McDowell of 33 Royal Avenue (opposite the Grand Central Hotel) for rings and other gifts.
By 1982 the premises appear to have been Apollo Window Blinds.
I think I have a clue for you though. I found this family in the 1911 census where there was a 20 year old Agnes McDowell who was a saleswoman and she had a brother Alfred. That might fit:
http://www.census.nationalarchives.ie/pages/1911/Down/Ormeau/Deramore_Avenue/211350/
Part of the family in 1901:
http://www.census.nationalarchives.ie/pages/1901/Antrim/Windsor/Edinburgh_Street/965981/
Alfred’s birth on 21.1.1887 in Belfast gives his parents as Charles McDowell & Margaret Cotter. So in the 1901 census the children are with their grandparents. Perhaps because Charles was dead? (Margaret is a widow in the 1911 census).
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1887/02574/1951667.pdf
1911 census tells you there were 6 children and all were still alive then.
Parents married in 1881:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1881/10999/8021360.pdf
Marriage for Angus in 1918:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1918/09725/5528616.pdf
Angus looks to have died in 1952 when he was living near Coleraine, Co Londonderry. His will should be in PRONI in Belfast and you might expect it to detail his children. There are several trees on Ancestry with the family. He looks to have had 3 children all of whom are dead now, but perhaps there are descendants? Contact the tree owners.
Probate abstract: McDowell Angus of Parkmount Carthall Coleraine county Londonderry commercial traveller died 5 May 1952 at Bann View Hospital Coleraine Probate Belfast 23 September to Anna Florence McDowell the widow. Effects £1653 15s. 2d.
Marriage for Alfred in 1915:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1915/09816/5564537.pdf
Likely marriage for Charles McDowell in 1918:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1918/09699/5518367.pdf
There’s an Ancestry tree for Alfred but there’s no information on any descendants.
A tree for Charles McDowell shows him as having died in Belfast in 1919 though I can’t find a death that fits that, and suspect it might not be quite right as he had 2 children. One was a Charles b 1919 who died in 2001 in Vancouver, and the other was Marjory McDowell with no dates. She is recorded as having died in London. Tree owner may be able to help you there.
I suspect Agnes never married.
Marriage for Mary McDowell to John Brownlee in 1915:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1915/09827/5568279.pdf
Annie married in 1908 to George Sargent:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1908/10061/5659306.pdf
Annie’s father Charles was noted as dead in the 1908 marriage certificate. There is a tree on Ancestry that indicates they moved to Canada in 1911 and George died in Ontario in 1954.
Angus looks to be your best bet at the moment for any descendants still in Ireland. Some trees show his death as being in Belfast but that does not appear to be correct. Both the PRONI wills site and GRONI death records show him living and dying in Coleraine which is about 60 miles north of Belfast. His death record gives his age as 66.