Longfield East RC baptisms start in 1846 so that might be too late. They are on-line on the nli site. Expect the spelling of the surname to vary. Include Mullin, Mullen, Mullan in your searches. Exact spelling wasn’t something our ancestors were bothered about.
The Edward Mullins in Griffiths looks to have been a farmer with 15 acres on plot 27. Does that occupation tie in with Margaret’s marriage certificate? Was her father a farmer?
I also note that next door on plot 28 is Mark Mullins with land but no house. So he probably someone else, but was likely related.
The Valuation Revision records show Edward’s farm passing to Michael Mullins sometime between 1860 – 1863 and by 1864 the tenant was James Gormley. So possibly Edward died around 1860. Are Mark or Michael family names by any chance? Mark’s land remained in his name till 1895 when another tenant took it over. I found a death for Mark Mullin bachelor & farmer on 14.1.1884, aged 84. He lived in the nearby townland of Laght.
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1894/05977/4699308.pdf
Informant was his niece Alice McCanny, also of Laght. This is probably her in 1901:
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Mullagharn/Laght/1742798/
There were a couple of other Mullin(s) farms in the townland who were probably related to Edward, and those 2 farms were still occupied by that family in 1901:
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Mullagharn/Garvaghullion/
Still 2 there in 1911:
http://www.census.nationalarchives.ie/pages/1911/Tyrone/Mullagharn/Garvaghullion/
Probate abstract for one of the sons who died intestate in 1918:
Administration of the Estate of the late Charles Mullen, Garvaghullion, Co. Tyrone, Sergeant RAF, who died 17 July 1918, granted at Londonderry 6 June 1919 to Charles Mullen, Farmer. Effects: £149 3s 10d.
Perhaps there are still Mullen families living there today. If so, a DNA comparison might be interesting if they were agreeable.