The Costello's built three main Castles, which were strongholds. One was Castlemore Costello, the second was at Kilcolman beside the old graveyard, and the third was at Tullaghaunrock. Costello’s were originally of the sect of Milesius but had retired to Normandy in France and lived there in the tenth and eleventh centuries. Returning to England they joined in the conquest of Britain, by William the Conqueror, and commanded much power and respect. Here they resided for another century. They accompanied King Henry II in the invasion of Ireland and having acquired the O'Gara lands in the barony of Costello, they spent many years in internecine disputes, mostly with the O’Conors, the Berminghams, the O'Garas and the MacDermots. This would indicate that they did not coexist too comfortably with their neighbours. The Annals of Ireland, or The Four Masters, records quite an amount of the Costello history until the sixteenth century.

By the middle of the fifteenth century the Costello Castle and Kilcolman Castle had been destroyed. The dwelling place of Edmund Costello was in Edmondstown (in the old house which belonged to the solicitor Malachy Muldoon, who wrote the play – The West's Awake). It still stands. Later Arthur Gorges Costello, the last of that family, built the Palace called now St. Nathy’s. He was buried in Urlaur Abbey churchyard in 1891. This land belonged to the Dominican Order and was sponsored by the Costello family about 1270. In the fourteenth century the Costello's gave fourteen acres of land in Belahaunis to the Augustinians to build their Abbey there. An ancient map found in Trinity College made in the reign of Queen Elizabeth. The Waldron Costellos lived in Bealahaunis and were a strong factor in that area. Alas the Augustinian Abbey closed this year.

There was Jordan Nangle, Gilbert Nangle and Colonel Nangle of Castile in Spain. This man was called the Castillion and it is supposed that this is where the name Costello originated. In the reign of Queen Elizabeth the extension of English rule throughout the western province was apparent. She appointed Viscount Theobald Dillon as collector and receiver of composition money for the province of Connaught. He put his legal education to good use. Within a short time Clan Costello had been virtually dispossessed of the their lands. A good deal of mystery surrounds the circumstances in which Theobald Dillon was invited into East Mayo by Mac Costello and given a free gift of a great portion of his land.

A more likely story is the fact that Garret Dillon brother of the first Viscount married the daughter of Shane MacCostello in the sixteenth century. There could have been an agreement due to this marriage alliance. This left the ownership of the 8,511 acres in Costello hands, with only a small yearly composition rent of £31/6/1 to pay Dillon. Three hundred years later this rent is mentioned in a Memorial Deed. So it appears that the Costellos were owners of that estate. It should be noted that when the final land settlement was agreed, it was with Costello and not Dillon the transaction was made. Another point to note is that down through the centuries, marriage alliances were made with various families and the name Costello spread throughout Connaught. Descendants of that great family are still in Connemara, like Seán Ó Coisdealbha and Máirtín Daibhi Ó Coisdealbha and their families. The name reached Achill Island, south Roscommon, Sligo, Athenry, where they married into the Birmingham family, the Mac Jordans of Gallen, the Clooneyquin Costellos and they made marriage alliances with the famous Clan of Terence McDonough MP Sligo.

The most well known of the Costello Clan was Dubhaltach Mac Coisdealbha the celebrated Rapparee who returned from exile in Spain with King Charles II around 1660. He had high hopes of regaining the lands they had lost to the Dillons. Like so many of his co-religionists his hopes were cruelly dashed. The Cromwellian settlement and previous settlements were not to be overturned. Dudley turned Rapparee and gathering a party of followers around him he carried on a vendetta of raids and burnings against the family of Viscount Dillon. There was a price on his head and he was proclaimed a tory and a rebel in 1666. He was shot dead by the soldiers of Dillon on the slopes of Sliabh Lugha, at Barnalyra (Barr na Laidre- the top/ height of the river fork,) beside the location of the new Connaught Airport.

According to tradition Dubhaltach was the brother of Tomás Láidir and they were sons of Jordan Boy Mac Costello, who lived in the old castle of Tullaghanrock. Dudley had a son -young Dudley who had a price put on his head by Dillon. Michael Rushe who was looking after him took him away in the night to Lisburn in Ulster. Here he passed as Rushe's son. He hoped that Dudley could come back some day to Tullaghanrock and claim his inheritance. Due to a disagreement with a neighbour they had to leave Lisburn in a hurry. Rushe heard of the strength and power of Dillon and of his influence with the English government. The Castles of his ancestors in Ballyhaunis, Airteach, Castlemore, and Kilcolman were in ruins. Tullaghanrock had gone into decay. The faithful followers of his house were scattered. Two years passed. Still Dudley did not know his name. At this time hurling was the great Irish sport of the day. At one of these matches Dudley was the hero of the day. The judge presented him with a yew caman or hurley stick with a silken tassle attached to it. Michael Rushe's heart nearly exploded with joy and he declared openly who the young hero was.

Next day young Dudley was on Tullaghanrock. (it is now a pile of ruins overgrown with moss and weeds, and is situated up Curry road off the main Ballaghaderreen Sligo Road) His father's horse was in Páirc an Stáil looked after by and old retainer and he found the Toledo blade worn by his father in the service of King Charles II. Dillon sent a keen swordsman to challenge him. They fought at Toby's Ford and Tobias Dillon was killed. Some months later Young Dudley fell from his horse and was killed at the pass at Bealach a' Dóirín (there was no town there as yet). This story and that of Tomáis Láidir are the most interesting of the Costello history. Dr. Douglas Hyde has the complete account of this tragic love story and the many verses of the song Tomás composed for Una Bhán in his book called 'The love Songs of Connaught’

The sister of Jordan Boy and Tomás L6idir, Margaret, married William Costello of Castlemore. It was at this time the blind Harper Turlough Carolan was earning his living be playing the harp at various ‘big houses’ and he composed many tunes for members of the Costello families. Edmond Costello's daughter married Arthur French of Frenchpark. These marriage alliances were beneficial to the Costellos. During the nineteenth century the Costellos do not appear to have taken part in the political life if the country. This was probably due to family matters. The estate was mortgaged to the hilt. Arthur Gorges Costello built the lovely elegant house at Edmondstown for his wife Diana Creagh. Finally the estate was purchased under the terms of the Land Act 1881. There were 269 tenants on the property. The purchase was £25,000. A.G. Costello had given 3000 acres of bog free to the tenants before this. He held on to the residence and home farm for some time longer. In 1885 the judges of the Chancery Division drew up a Memorial of a Deed of Conveyance. It was the beginning of the ownership for the tenants and the end of an era for the Costellos.