My Grandparents birth records

The place to start is with the marriage of Maria & John and there's a good match on the free IrishGenealogy/GRO website :

Maria Curran & John O'Brien 19th January 1927, marriage took place in the RC church in Arklow, the groom's age given as 24 occupation tailor and residence looks like Chapel Ground Arklow, the bride's age as 18, and her occupation home duties, and residence R..gath, Woodenbridge, their fathers as Michael O'Brien tailor, and Matthew Curran Blacksmith. The witnesses are Patrick O'Brien and Annie Byrne.

probable birth for Mary Elizabeth Curran 9th December 1906 - father's name and occupation matches. Mother's name Anne Adams, place of birth of the child and residence appears to be Rostigath, in Arklow sub-district.

Here's the Curran family on the 1911 census at Rostygath townland.
and the O'Brien family at Chapel Grounds, Arklow in 1911.

The O'Brien side was much more difficult, but eventually found the following :

1901 Census - Brien household, Raheennaskeagh Upper, Co. Wexford - includes Michael Brien master tailor, wife Mary and thier children.
Birth for one of the younger children Ellen in 1899 gives father's occupation as Tailor, and mother's surname as Kavanagh
following on from that was able to find the birth for John on the 6th December 1902, at Old Chapel Ground, Arklow, father Michael O'Brien occupation Tailor, mother Mary Kavanagh

The more recent information is not as readily available, mainly due to privacy restrictions e.g. the most recent census online is for 1911, birth records on the GRO/IrishGenealogy website covers records up to 100 years ago, marriages up to 75 years ago and deaths over 50 years ago.

The version of the BMD Index on FamilySearch covers up to 1958 for the 26 counties, and from about 1927 includes mother's maiden surname, which in combination with the registration district can help narrow down likely births between these dates - there are some good possible matches showing up for the combination of O'Brien & Curran - see this link. Watch for those listed in Rathdrum registration district as this covered the town of Arklow. You can use the references on the index, year/quarter volume & page etc., to order research certs costing €4 for these records to check the full details.

Even though many people are now ex-directory or do not have a land line, it can be useful to try online phone directories, but I think this would be difficult for the O'Brien side, but might be useful with the less common Curran surname.

Now that the details are posted here, maybe someone related could find this topic at some stage...

p.s. found some directory listing in Thom's 1930 that look connected to your O'Briens :

O'Brien & Sons, 23 Chapel Ground, Arklow - Tailor
Patrick O'Brien, 16 Lr. Main St., Arklow - Tailor

and in Thom's 1943 :

O'Brien & Sons, 23 Chapel Ground, Arklow - Tailor
Patrick O'Brien, 16 Lr. Main St., Arklow - Tailor

The informant had to be someone present during the birth, so Margaret was most likely helping out with the birth.
The 1911 census shows a probable match for Margaret Long living with her husband and two children at 18 Back Street Arklow.

Back Street, Arklow is still there, and appears on maps back to the 1840s - see c1890 OSI map. The name possibly derives from it being located adjacent to the Main Street, so running along the back of these buildings. There's a similarly named Back Lane in Dublin City which runs adjacent to High Street.  Back Street Arklow is very close the Chapel Ground which was mentioned earlier as an address for your O'Briens.