Complex sentence: Still intending to take up the offer of work in Ireland, he had been befriended by John Erskine, 22nd Earl of Mar while abroad, the Earl persuaded Gibbs to remain in London, offering Gibbs his first commission, alterations of his house in Whitehall.
Split into simpler sentences: Still intending to take up the offer of work in Ireland, he had been befriended by John Erskine, 22nd Earl of Mar while abroad. The Earl persuaded Gibbs to remain in London, offering him his first commission, alterations to his house in Whitehall.