Fitzgerald, like his brother Charles and unlike the rest of his family, but with the encouragement of his relative the Duke of Richmond, was an admirer of Pitt.
On his return he stepped into a power vacuum created by his brother the Duke of Leinster’s opposition to the Union. Rather than contest county Kildare against a friend of government, Leinster agreed to Lord Robert’s offering, as a friend of the Union and of ministers, thus, as Lord Grenville observed, securing his family interest. Leinster subsequently admitted that Lord Robert was ‘the best locum tenens’ for his son. Informing Pitt of this scheme, 27 June 1800, Fitzgerald added that his brother made but one condition, ‘that of vacating the seat at any time that a change of ministers and measures may present him with an opportunity of more effectually supporting those with whom he has chiefly been in the custom of acting’. Castlereagh welcomed the scheme, declaring that ‘there could not be a more desirable candidate’. In October 1801 Fitzgerald applied to the Addington ministry for support in accordance with this plan, taking care to applaud the peace preliminaries, and was promised it and duly elected.
A few weeks later he became minister to Portugal; he had applied for this unsuccessfully in July 1800, but had been earmarked for it by January 1802. In the preceding March, on receipt of a civil list pension of £800 a year, he had complained to Pitt that it was inadequate for his services and asked him to recommend him for a rise, or for promotion in the foreign line. At that time he was still a ‘warm admirer and advocate’ of the outgoing Pitt administration. He was unable to display his attitude, professedly favourable, to its successor, being abroad from October 1802, when he appealed to Addington to upgrade him to an ambassador to meet his expenses. In fact, he took his seat during a short period of leave, 15 June 1805.
He had not cared for his mission, and as early as March 1803 appealed to Wickham to find him an Irish place compatible with Parliament. On Pitt’s return to power he had asked that minister for his deceased brother’s Irish ribbon, 12 Dec. 1804, but had to resign himself to the fact that it was never given to a commoner. In December 1805, being in financial difficulties, he applied for leave of absence, to live in Ireland for a year.
