I know there's a published history of this parish, written ??80/100 years ago - i got it in my local library in North Dublin.
Unfortunately I can't remember the author, a catholic priest who wrote histories of Blackrock and maybe other parishes.
If you can track it down, it laid out in great detail, when churches were built, made in to parishes, divided etc.
The Diocesan Directory 2006 gives the following
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DONNYBROOK(Sacred Heart) constituted 1876 from Irishtown, Ringsend and Donnybrook .... so although built in 1865, it was chapel of ease, not a parish, until 1876/77.
HADDINGTON ROAD (St Mary's) constituted 1876 from parish of Irishtown, Ringsend, Donnybrook and Sandymount
IRISHTOWN ... nothing
RINGSEND (St Patrick) constituted 1905 from Sandymount within the original parish of Irishtown, Ringsend and Donnybrook
SANDYMOUNT (St Mary Star of the Sea) constituted 1851
I suppose they used the old St Mary's in Donnybrook Village until it got too decrepit - from Catholic Emancipation in 1829 more Churches were built ... perhaps Sandymount first 1851 ... then perhaps the parish was split 3 ways in 1876

(In the suburbs in 1970s this splitting was still going on, the old Parish of Baldoyle now has at least three parishes ... )
I hope this hasn't confused things still further!!
eadaoin