Thomas Brassey was father in law to Matilda Brassey, sister of 5th Baron Clanmorris.
- The O'Donel estate on the island of Achill was bought by Trustees of the Achill Mission in association with Thomas Brassey, William Pike and Samuel Holme, who each paid £2,333.6s.8d. for small estates on the island in the early 1850s. The Mission's share was 23,452 acres for which they paid £10,500 . In 1876 the Mission is recorded as holding 19,155 acres, valued at £1,011 in county Mayo. By March 1916 the Achill Mission had accepted an offer from the Congested Districts' Board for the purchase of their estate, which the Board took over in 1921.
- Thomas Brassey bought 2,592 acres on Achill island, barony of Burrishoole, county Mayo, part of the estate of the O'Donels of Newport for £2,333 in the early 1850s.