The authority on Irish family genealogies is Dubhaltach Mac Firbisigh who compiled his history of the prominent families of Ireland in his "Leabhar na nGenealach" meaning the Book of Genealogies now commonly referred to as the "The Great Book of Irish Genealogies". Mac Firbisigh compiled the book from other older books and manuscripts many now long lost, he started about 1645 and finished in 1666. In this book Mac Firbisigh included the Costellos or Mac Coisdealbhaigh of Mayo

He has the family tree included from Miles Bregach Mac Coisdealbhaigh. He says that Miles was the son of Philip, who is the son of William, who is the son of Jocelyn De Angulo. So the family is said to have started with Jocelyn De Angulo. He came to Ireland with his sons Philip, William and Gilbert De Angulo. However the Irish called these soons MacGoisdelbh (sons of Jocelyn) which over time became MacCoisdealbha and was mistranslated into English as Mac Costello. The Mac was often dropped so you just end up with Costello or Costelloe.

Jocelyn was said to be from Angle, Pembrokeshire, Wales and this is where De Angulo comes from. Jocelyn De Angulo was listed as one of the 50 Knights who came with Hugh De Lacey to Ireland in 1171. He was enfeoffed with the Barony of Navan and the lands at Ardbraccan (just west of Navan) in County Meath by Hugh De Lacey (Henry II representative in Ireland at the time). Gilbert was given the Baronies of Morgallion and Ratoath. Gilbert and his brothers Philip and William were outlawed for treason in 1195 against King Richard I (The Lionheart). The brothers fled, in 1195 Gilbert went west and took service under the King of Connacht, Cathal Crobhdearg Ua Conchobair, who also gave him lands in Máenmaige (modern day Barony of Loughrea Co Galway) Gilbert was pardoned by King John of England in 1206 and he confirmed the grant of land which Cathal had given him and granted him some additional lands. Gilbert never regained his lands in Meath. Gilbert helped Cathal build a castle Caeluisce near Ballyshannon, Co Donegal in 1212. The castle was attacked and burnt in 1213 with Gilbert dying in the attack. Gilberts line is thought to have died out in the male line at least, as Mac Firbisigh finds no more trace of them.

Philip was also pardoned in 1206 and inherited Navan from his father Jocelyn. Philip is said to be the ancestor of the Nangle family, this name was also changed to Nagle around Mallow in Co Cork.

William is said to be the ancestor of the broader Costello clan and is also an ancestor of the Mac Jordan Duff, McPhillips and Waldrons of Mayo. He held lands in Meath which were returned to him once he was pardoned in 1206. Williams grandson was Miles Bregach
Mac Coisdealbhaigh by his own son Philip. "Bregach" signifies an area or population in Meath/north Dublin.

Miles MacGoisdealbh was made lord of Sliabh Lugha (now part of the Barony of Costello Co Mayo) by the De Laceys. He was also mentioned fighting in Co Leitrim for the Lord of Navan. He ended up making his home in Castlemore (now just outside Ballaghadreen, Co Roscommon) he died in 1259 his wife was buried in Boyle Abbey Co Roscommon. The Costellos went on to control and own large tracts of east county Mayo in an area now known as the Barony of Costello. Many of these medieval Costellos were buried in Urlaur graveyard which was part of a monastery they had founded. The Costellos seem to have lost out on huge tracts of their lands to their neighbours the Dillons around the time of the reformation. They seem to have held on to Castlemore outside Ballaghadreen. In the mid nineteenth century they seemed to abandon Castlemore and built a brand house on the east of Ballaghadreen called Edmondstown.