Since I last wrote I have been doing a lot of research into the Shoulder family in Louth and Armagh as my one of my great grandmothers is a Margaret Shoulder, and her father my great great grandfather is Arthur Shoulder from Kilcurry, Co. Louth just north of Dundalk.
I found the baptisms of Owen in 1858 and James Shoulder in 1860 in the RC Parish of Upper Creggan, Armagh (which covered 10 townlands in north Co. Louth as well as townlands in south Co. Armagh) and their parents were Laurence Shoulder and Anne Murphy. Owen's godparents were Thomas Brennan and Catherine Murphy, and James' godparents were Francis Traynor and Bridget Murphy. Catherine and Bridget Murphy were probably their aunts and sisters of Anne. Their baptismal records do not say where they lived. I could not find a marriage record for Laurence and Anne, but there are gaps in the church records and missing records (I can't find a baptismal certificate for my Arthur Shoulder born circa 1805).
I looked at the Laurence Shoulder that Elwyn mentions, who was renting a house, buildings and lands from Denis Kelly in Carran, Creggan, Co. Armagh in Griffiths Valuation circa 1865. Carran is a townland just north of Crossmaglen, Co. Armagh (I have ancestors from here too) and very close to the border with Co. Louth. There was also a Laurence Shoulder in Carran in 1827 on the Tithe Applotments. I found the 1818 marriage of a Laurence Shoulder to Anne Callaghan in the RC Parish of Creggan, and I think this is the Laurence who resided at Carran and who died in 1870 aged 88. Between 1820 to 1830 there were baptisms in Creggan RC Parish of six children to this Laurence and Anne: James, Peter, Bridget, Mary, Michael and Rose. I didn't find the baptism of a Laurence to this family, but it is possible there could have been one, or that this family are close cousins to your Laurence.
However I did find the baptism of a Laurence Shoulder on 20/09/1833 to a Peter Shoulder and Jane Byrne in the nearby Dundalk RC parish (godparents Daniel Mires and Mary Hamill). They lived in Slieve just north west of Dundalk beside Roche and Kilcurry ,where my Shoulder family are from, and only a couple of miles from the Armagh border. This Peter is also on the Tithe Applotments for Slieve which is in Co. Louth. The following children were also baptised to them: Michael (1822), Bridget (1827), Catherine(1829), Owen (1831) and Mary (1836). The death of a Peter Shoulder aged 80 years is registered in Dundalk district. I think he could be the grandfather of your James and Owen Shoulder.
Is this of any help?