Rosanna Cahill / Rose Nagle

I believe the place name is Daingean.

Geashill (in Irish, Géisill) is the name of a town in Offaly, and is also the name of a townland which covers much of the southern part of the town.  Dalgan is the name of a separate townland which covers much of the northern part of the town.  You can see more about those townlands (with maps in each case) at these links:

https://www.townlands.ie/offaly/geashill/geashill/geashill/geashill/

https://www.townlands.ie/offaly/geashill/geashill/geashill/dalgan/

Those townlands and the surrounding area are located in a civil parish which is also named Geashill.

Daingean, formerly called Philipstown, is the name of a town not far to the north of Geashill, and Philipstown is also the name of the Catholic parish which includes all of these areas (including Geashill).  St. Mary’s is still in operation, but not as a cathedral.  I’m not sure what the name of its Church of Ireland parish is now, but St. Mary's is apparently part of the "Geashill Union of Parishes" (probably a consolidation due to the decline in the number of Protestants in the area).  You can see St. Mary's north of the main road through town in a Google Earth image at this link:

https://2ua.org/irl/l/dalgan/map/

They also have a website where you can see a picture of the church and find contact information:

https://meathandkildare.org/findachurch/clonmacnoise/geashill-union-of-…

The baptism of Rose Cahill actually took place in the Catholic Parish of St. Mary's in Dublin city, date, parents and sponsors match the details as posted : baptised 16th July 1822 parents Lewis Cahill and Rose, no mother’s maiden surname is noted, residence for the family is recorded as 'Malbro St.' (Marlborough) which is the same street where the Catholic Pro-Cathedral was under construction at the time of Rose’s birth.

Two years earlier there’s a baptism of an older sister to Rosanna, Eliza in the same parish on the 28th September 1820, this time the residence is noted as 'Liffy St.' (Liffey) coincidently the same street where one of the earlier chapels for the parish of St. Mary was based.

Just about a year and a half earlier is a promising marriage for the couple – date is  12th April 1819 in the same parish – Lewis Cahill and Rose Mc Laughlin… so not Nagel (Nagle).unfortunately.  The forename Lewis was not common at the time so as it's the same parish as the two baptisms and not long before the baptism of the eldest child I believe a very probable match for this couple and their marriage.

See attached register extracts for baptisms and marriage, all St. Mary’s R.C. Dublin city.

Baptism Rose Cahill 16 July 1822.jpg

Baptism Eliza Cahill 28 Sept 1820.jpg

Marriage Lewis Cahill & Rose Mc Loughlin 12 Apr 1819.jpg