Weld’s name stems from his great-grandfather Nathanael Weld’s close friendship with Sir Isaac Newton, and as such both his grandfather and father are also named Isaac. His father is a close friend of Charles James Fox. His sister marries George Ensor, and their half-brother is Charles Richard Weld, traveler and author of A Vacation Tour in the United States and Canada (London, 1855), which is dedicated to his brother, Isaac. He is sent to the school of Samuel Whyte at Grafton Street and from there to another private school Barbauld at Palgrave near the town of Diss in Norfolk. From Diss he proceeds to Norwich as a private pupil of Dr. William Enfield. He leaves Norwich in 1793.
In 1795 he sails to Philadelphia from Dublin and spends two years traveling in the United States and Canada, partly as an adventure and partly as research into suitable countries to which the Irish can emigrate. He visits Monticello and Mount Vernon and meets George Washington and Thomas Jefferson. He travels on horseback, by coach and by canoe in Canada with local native guides. He returns in 1797 “without entertaining the slightest wish to revisit it.” He finds the Americans to be obsessed with material things and prefers Canada. His published Travels (1799) quickly goes into three editions and is translated into French, German, Italian, and Dutch.
Weld writes on slavery that “there will be an end to slavery in the United States…[as] negroes will not remain deaf to the inviting call of liberty forever.” With regard to Americans in general, he states, “Civility cannot be purchased from them on any terms; they seem to think that it is incompatible with freedom.” On Washington, D.C., he writes “If the affairs of the United States go on as rapidly as they have done, it will become the grand emporium of the West, and rival in magnitude and splendour the cities of the whole world.”
Weld visits Killarney, navigates the lakes in a boat he made from compressed brown paper, and publishes Scenery of Killarney (1807), illustrated with his own drawings. He is also well known for his drawings of American life and, in particular, the Niagara Falls.
In May 1815 Weld sails from Dún Laoghaire to London in the 14 horsepower (10 kW) steamboat Thames, the first such vessel to make the passage. He compiles the Statistical Survey of the County of Roscommon (1838) for the Royal Dublin Society, of which he is Honorary Secretary and Vice-President. In later life, he spends much time in Italy and particularly Rome, where he develops a friendship with Antonio Canova.
Weld is part of the Weld family of New England. His ancestor, Thomas Welde, is a Puritan minister from Suffolk, England who is one of three brothers who emigrated to Roxbury, Massachusetts in 1632. His great-great-grandfather, Thomas Weld, helps to publish the Bay Psalm Book, the first book published in America. His great-grandfather, Nathaniel, is graduated at Harvard College. He leaves Massachusetts for Kinsale and then Blarney Castle, County Cork, in 1655 to be a Puritan Chaplain with Oliver Cromwell. He later moves to Dublin.