Surnames began in the 10th century, and were fully entrenched by the 12th
century. The gaelic names were altered under the Penal Laws, and the altered
versions remained until the time of the Gaelic League in 1893, when many,
especially in the western counties, reclaimed their Gaelic names.
Gilligan comes from MagGiollagain (accented "a" at the end), a leading sept
from County Derry. It is sometimes Gillan. There is an area in north Derry,
near Lough Foyle called Magilligans Strand. They are of Tir Eoghain, who
lived in Tyrone and Derry.
O'Gealagain (also accented "a" at the end) is a sept of UiFiachrach in Sligo,
and the name is now often Gilligan. This sept is also in Cavan as Galligan
and sometimes as White. The UiFiachrach are from the Muardhe north of Mayo
and in Sligo.
Gill comes from Mac an Ghoill, which is an abbreviation of many names which
begin with MacGioll.
Gilliland is totally unrelated and is now McLellan.
Gilgan is unrelated to the MagGiollagains, and comes from north Leitrim and
Sligo from the name O'Giollagain.
The Giolla part of the name means servant, follower or lad.
Sincere thanks to my good friend, Sharon, who submitted this information which can be
found in the book "A Guide to Irish Surnames" by Edward MacLysaght.