The O’Donnellans were a sept of the Ui Máine. They claim descent from Domhnallán,lord of Clan Breasail. 

Domnallan was the son of Maelbrigdi, a member of the Ui Maine dynasty. His pedigree in the Leabhar Uí Maine is given as Domnallan mac Maelbrigdi mic Grenain mic Loingsich mic Domnallain mic Bresail mic Dluthaig mic Fithchellaig mic Dicholla mic Eogain

However John O'Hart claims "Cathal, brother of Inrachtach, who is No. 98 on the "O'Beirne" pedigree, was the ancestor of O'Donallain; anglicised Donnellan, Donalan, Donelan, and Donlan"

Ballydonnellan Castle was the family seat. The heads of this family were the chiefs of Clann Bhreasail, the district lying between Loughrea and Ballinasloe.

The original castle is said to have been erected as early as 936 but this, more likely than not, was probably a ring fort. Another castle built on the same site was destroyed by fire some time before 1412 when it was rebuilt by the O'Donnellans.

During the seventeeth century the family added a long two-storey house to the side of the castle. During the following century the castle was renovated and extended to form an end pavilion wing, and a matching wing was added to the other end of the seventeenth-century structure. The wings were redesigned after 1787 when the façade of the central range was altered with a four-bay pedimented break front. One of the wings was destroyed by fire before 1913 but the entire house was in ruin by 1978. This is located about two miles north of Kilreekill.

They are chiefly known as ollamhs or poets, many of whom are mentioned in the"Annals of the Four Masters," the "Annals of Connacht" etc.

The best known of them was Brian Mac Owen O’Donnellan (fl. C. 1610), poet to MacWilliamof Clanricard, whom Douglas Hyde describes as one of the last of the classic poets.

His contemporary, Rt. Rev. Nehemiah Donnellan (d. 1609), Protestant Archbishop of Tuam, translated a great part of the New Testament into Irish. He was the direct descendant of Chiefs of the Name, of Ballydonnellan, and ancestor of the Donelans of Sylanmore, Tuam. The majority of the Donelans of this line reverted to the Catholic faith.

The name is quite common to-day in Co. Galway and also in the adjacent counties of Clare and Mayo.

It is also spelled Donlan,Donlon, Donnellon, Donlin, Donelin, Donelan among other variants. In Irish it is Ó Domhnalláin, indicating descent from the Domhnallán mentioned above.

ref: 

http://www.libraryireland.com/Pedigrees1/Donnellan-1-Heremon.php & http://freepages.genealogy.rootsweb.ancestry.com/~dgarvey/Griff_Garvey/Clann_Cathail.html

http://www.ballyd.com/ 

https://en.wikipedia.org/wiki/Donnellan