1. Any person named O'Reilly living today would only have an average of 30 direct lineal male ancestors named O'Reilly. The average is three per century with a gap of about 33 years between each generation. The accuracy of this general rule usually works out quite well, as in e.g. the well publicised pedigree of the O'Reilly's of Heath House, County Laois given below. This fact is encouraging to any O'Reilly researching their ancestry as they would already know at least 3 or 4 direct male ancestors (father, grandfather etc.) which leaves only about 26 or less to go.
26th generation= Myles John O’Reilly born 1860 25th generation= Myles George born 1829 24th generation= Myles John born 1780 23rd generation= Dowell born c. 1747 22nd generation= Myles of Tullystown born 1708 21st generation= Captain Brian of Ballenrink born 1676 20th generation= Colonel John of Garrirobuck born 1644 19th generation= Myles born c. 1610 18th generation= Brian born c. 1575 17th generation= Sean born c. 1540 16th generation= Lord Eamon of Kilnacrott born 1508 15th generation= Lord Maol Mordha born c. 1475 14th generation= Lord Sean born c. 1440 13th generation= Lord Cathal born c. 1405 12th generation= Lord Eoghan na Feisoige born c. 1370 11th generation= Lord Sean born c. 1335 10th generation= Lord Philip born c. 1300 9th generation= Lord Giolla Iosa Ruadh born c. 1265 8th generation= Lord Domhnall Mor born c. 1230 7th generation= Lord Cathal na Beithighe born c. 1190 6th generation= Lord Annadh born c. 1140 5th generation= Lord Cathal na gCaorach born c. 1110 4th generation= Lord Gofraidh born c. 1080 3rd generation= Lord Cathal Mac na hOidhche born c. 1050 2nd generation= Lord Cu Chonnacht born c. 1020 1st generation= Lord Airghiallach O'Reilly, the first person with an O'Reilly surname. He was born c. 990
2. In 1891 the number of males bearing the O'Reilly surname in Ireland was roughly 15,000, about 0.3% of the total Irish population. Assuming this percentage holds roughly true in preceding years,then if you extrapolate backwards the following table shows the number of O'Reilly males living at any given period in Irish history.
Year/Irish Population/Male O'Reillys
1300/ 1,400,000/ 4,200 1400/ 700,000/ 2,100 (Population fell due to Black Death) 1500/ 800,000/ 2,400 1600/ 1,000,000/ 3,000 1700/ 1,900,000/ 5,960 1800/ 5,395,000/ 16,185 1881/ 5,174,000/ 15,522
So at any given time before 1700 there are only a few thousand male O'Reillys living, which again makes it easier to track their descendants.
3. The O'Reilly clan has probably the most complete surviving genealogy of any Gaelic family from it's origin up to the 1700's, so if you can get back as far as this point the research becomes much easier, instead of more difficult as often happens with a lot of Gaelic surnames. I have worked out the genealogy of nearly every O'Reilly up to 1700 so if you need any info up to that point fell free to message me, but not anything after that date.