Of Maunsell all I know is this reference from "BARTON HALL, HEREFORD: A HISTORY" by David Rawson (BHR)
For the next twenty-two years, one of the leading Brethren was Thomas Maunsell. He was a gifted teacher, trained in the law, who had come from Ireland in 1850 and had been meeting with the Brethren at Bridge Street. He was an enthusiastic and inspiring man and the numbers in the fellowship increased. The morning meeting commenced at 10.30 a.m., and would often continue until one o’clock The Sunday school work of the assembly was begun primarily by him. In his southern Irish brogue he would often address the Sunday morning meeting for an hour or more. Young people must have loved him for the Sunday School was reported to be the best attended in the city. There is a record from November 15th, 1850 ‘...that the children of the saints be invited to meet from three to four o'clock and Mr. Maunsell will undertake for the present to instruct them in the Truth’.He died on 6th August 1880 aged 73.
Following Andrew Miller, Rowdon says that Maunsell came from Limerick (Origins, 96). Following Stoney, Neatby has Maunsell (spelt as 'Mansell') attending a ‘Powerscourt’ conference in 1838 (History, 39). In fact, this was the Clifton conference in that year but Maunsell’s attendance is independently attested in another account (see BHR viii  6) where his first name is given as ‘Robert’.
In a fragment (c.1838) in the JND scrapbook in Manchester (CBA.JND/1/1/16), Maunsell is named as being jointly responsible for the Epistle to the Hebrews in a proposed translation of the New Testament. (See Elusive Quest, 276).
Tim Grass (Gathering to His Name, 60) refers to a letter from Maunsell (16 July 1842) to JND. It is in the CBA.JND/5/164 where it is said to be concerned with ‘numbers of Brethren in Scotland and Northern England, asking if Darby knows of any Christians in France or Switzerland who would look after the sons of a Mr. Web who wants them to study abroad and stating his intention to join Darby in Switzerland.’
Two years later (Feb 1844) T. Maunsell is reported to be ‘at Bath’ (Simple Testimony i  248) which is where B W Newton (in a recollection some fifty years later) remonstrated with Ma[u]nsell about biblical interpretation (CBA7060 Wyatt MS Book vi p.30 verso).
Rawson’s account of Hereford implies that Maunsell began to take a leading part in 1858, but we should perhaps note that some years before that, his pamphlet (What was I that I could withstand God?) was published (Hereford 1852) by William Yapp who sent a copy with a covering letter to JND in March 1852 (CBA.JND/5/304) Timothy Stunt