The late Baroness Thatcher was, for a variety of reasons, no fan of the Irish; equally, neither are a number of Scots fans of hers.
Nevertheless, it appears that on her father’s side she was a descendant of Colonel Sir John William O’Sullivan, an Irishman and Quarter-Master General to Bonnie Prince Charlie in his 1745 insurrection against the British state.
Source: Charles Moore, Margaret Thatcher, The Authorised Biography, Volume One, Not For Turning, Allen Lane, 2013, p 622.