15.10.1799 meet Fitzgerald (ex PS) / 25.9.1800 ma. Fitzgerald at mrs Plunket's / 14.2.1802 Fitzgerald at John King's / 22.5.1804 3 Fitzgeralds at H Rowan's / 27.9.1804 Fitzgerald at H Rowan's / 11.3.1805 Fitzgerald calls (& mrs Rowan) / 15.3.1805 at H Rowan's / 21.5.1805 again / 4.8.1805 calls (& H Rowan) / 1.3.1816 dine at Curran's. Conference; Fitzgerald, pp.20 / 20.10.1817 Fitzgerald calls (not in) / 21.10.1817 again / 22.10.1817 again / 23.10.1817 calls / 24.10.1817 again / 25.10.1817 again / 27.10.1817 dines / 28.10.1817 calls, & W Curran / 4.11.1817 at Curran's funeral / 7.3.1826 at La C Lamb's / 13.8.1833 at Hume's
ex PS may have referred to the Philomath Society which Godwin was a member of 1793-1796. William Thomas Fitzgerald DNB 1759-1829 was not likely to have been a member, though his song The Sturdy Reformer (1792), while intended as satirical could have been taken as seditious. Although the DNB says he was entered as a member of  the Inner Temple his name is not in their Admissions Database. He was presumably the William Fitzgerald of Grays Inn whose Ode to Captain Cook was published in 1780. More likely as a Philomath Society member was William Fitzgerald (QV* in Society for Constitutional Information dataset).
Major Fitzgerald in 1800 could have been Augustine Fitzgerald Hist of Parl 1765-1834 on half pay of 117th foot / James Fitzgerald Major 3rd Foot Guards 4.10.1799 / Maurice Fitzgerald Hist of Parl 1774-1849 the Knight of Kerry, major in Kerry militia 1797 / maj Thomas Fitzgerald of Snowhaven married 1786 miss Latten, subscr 1798 to D R O'Conor, will PCC 1811
The later Fitzgeralds at Hamilton Rowan's and Curran's may have been Lord Henry Fitzgerald Hist of Parl 1761-1829 who lived near London and was said to be "tinctured with the principles of his brother" the rebel Lord Edward Fitzgerald DNB 1763-1798. Maurice Fitzgerald the Knight of Kerry (see above) though pro-government at the time of Anglo-Irish Union 1800 and a Lord of the Treasury thereafter became disillusioned with the failure of Catholic relief and went over to opposition around 1807. Edward Fitzgerald DNB & Dict Irish Biog c1770 - c 1807 was a United Irish leader of 1798 who according to DIB wasin England 1800-3 and was pardoned in 1803 but died in Hamburg in about 1807 while the DNB suggests he was in Hamburg from 1800 to his death (see also O'Byrne). There was also N A Fitzgerald the mother of two of Curran's sons (see Bodleian Abinger c7 f32-3), I don't know if she was still alive when Curran died but her son Henry Grattan Curran Dict Irish Biog 1800-1876 was known as Fitzgerald until he adopted his father's name in 1828 and the other son was probably the writer of a letter Bodleian Abinger c12 f15 concerning his father's burial which makes him a likely candidate for the 1817 entries at least. Crofton V Fitzgerald married in 1813 Harriet the daughter of Archibald Hamilton Rowan DNB 1751-1834. There were plenty of other Fitzgeralds around so none of these identifications are strong yet