In the Irish Census 1901,
6,882 people recorded with the family name Sheehan. 1,334 recorded with the family name Sheahan.
1,350 Sheehan in Co. Kerry, 224 Sheahan.
3,465 Sheehan in Co. Cork.
620 Sheehan Co. Limerick.
In the Irish Census 1911,
6,665 people recorded with the family name Sheehan, 1,185 recorded with the family name Sheahan.
1,341 Sheehan in Co Kerry, 196 Sheahan.
3,292 Sheehan in Co. Cork.
649 Sheehan in Co. Limerick.
I have also come across the name Sheehan mis-transcribed as Skehan, 296 in Ireland in 1901 and 285 in 1911 and also as Skeehan.