From "History Ireland" by Michael Laffan, head of the School of History in University College Dublin.
No mention of any invasion threat.

it was agreed that a boundary commission would decide the border between the two parts of Ireland. It is significant that the treaty split centred on questions of sovereignty and the oath of fidelity ('allegiance') to the king rather than on the question of partition. Few Dáil deputies discussed the matter. Either they felt that partition was already an established fact and that nothing could be done, or they assumed that the boundary commission clause would take care of the question. Some people were later embarrassed by this omission and tried to rewrite the record.


The treaty was supported by narrow majorities in the Irish cabinet and the Dáil, and in January 1922 Collins formed a provisional government. De Valera went into opposition, but the strongest opposition to the treaty came not from politicians but from elements in the IRA. Some soldiers were unwilling to accept civilian authority. Despite elections in June 1922, which revealed the popularity of the treaty (78 per cent of the first-preference votes were for candidates who supported it), civil war broke out soon afterwards.


The resulting struggle degenerated into a bloodier and more savage conflict than the recent war against the British, and both sides resorted to atrocities. But there was no swing of opinion against the government as had happened after 1916 and in 1919–21, and ultimately the republicans laid down their arms.


The civil war also ended southern concern with Northern Ireland and it brought to an end Collins's attempts to destabilise Craig's government in Belfast.


The civil war was only one factor among several that allowed time to elapse before the boundary commission was established, and not until late 1925 was it ready to complete its report. The chairman (South African jurist Richard Feetham, who was appointed by the British government) had the casting vote, and predictably he took a conservative and narrowly legal view of the changes that might be made to the border. Despite the hopes of the Irish delegation in the treaty negotiations, and despite the fact that one third of the population of Northern Ireland wished to join the Free State, the proposed amendments were minimal. To the shock of nationalists, it was even suggested that the Free State should hand over some of its territory. Ultimately the three governments decided that the border between North and South would remain unchanged.
http://www.historyireland.com/20th-century-contemporary-history/the-emergence-of-the-two-irelands-1912-25/