While I don't have any connections to the Boyles, Gallaghers or O'Briens from Donegal, I have some suggestions about how you may be able to find out more information about them.
This would be through the Catholic Church parish registers in Letterkenny and Falcarragh.
I am assuming that your ancestors were Roman Catholic.
The Mormon Church has microfilmed the Letterkenny Catholic parish registers. These registers go back to the year 1851. If Eugene and Sarah were married before 1851 in the Letterkenny parish, their marriage will not of course be in the registers, but there could very well be the baptism records in the registers for any of their children born in Letterkenny or surrounding area from 1851 and later.
The microfilm of these church registers can be ordered from your local Family History Center.
Please see below for information and microfilm numbers of the registers:
Parochial registers of Conwal and Leck , 1851-1962 Authors Catholic Church. Parish of Conwal and Leck (Donegal) (Main Author)
Notes Some writing is faded and illegible. The Catholic parish of Conwal and Leck, now called Letterkenny, includes the civil parish of Conwal and part of Leck. The Cathedral of St. Eunan, Letterkenny, is the seat of the bishop of the Diocese of Raphoe. There are chapels in Letterkenny and Kilpheak in addition to the Cathedral.
Baptisms, 1851-1924; marriages, 1857-1962; banns, 1853-1918.FHL BRITISH Film 1279203 Items 1 - 6
Banns, 1853-1918; baptisms, 1853-1898; marriages, 1854-1921.FHL BRITISH Film 1279236 Items 22 - 26
Baptisms and marriages, 1854-1880.FHL BRITISH Film 926208 ________
I'd suggest ordering all three films to see if you can locate any records for your ancestors in the church registers.
The Mormon Church unfortunatley has not microfilmed the parish registers for Falcarragh. One of the Catholic Churches in the Falcarragh area has registers that go back to 1849. What you will have to do is write to the parish priest and ask that he search the registers for either marriage or baptism entries concerning your ancestors.
Address your letter to:
Father Connell Cunningham, PP Falcarragh, Letterkenny, County Donegal, Ireland
If you write to Father Cunningham you'll want to send a donation for the poor of the parish as a way of thanking him for help with your genealogy. 10-15 Euros would be a very good donation. You can order the Euros at your local bank and then enclose the money with your letter. If you don't feel comfortable sending currency through the mail, you would then have to obtain an international money order, which can be very expensive.
If your ancestors belonged to a Protestant denomination in Donegal, please let me know and I'll see if I can find the names and addresses of the ministers in Letterkenny and Falcarragh.
Don't heistate to write if you have any questions.