"Is Britain prepared to send in armed forces? No Irish government is going to sit still for that."


I very much doubt if 'Britain' is contemplating invading the Irish Republic, the Irish government has no need to sit/stand/worry about that.


Regarding deploying UK troops in Northern Ireland, if the need arises I see no problem, it's well established that there are plans to use the army post-Brexit in the UK if necessary. I sincerely hope that there are military/security service staff in the North of Ireland already as there have been mutterings about nationalist violence and this would seem to be a sensible precaution.