But Ireland has said many time they do not want NI! Possibly because of the huge Social benefits bill they would have to shoulder;

current NI £5.6bn bill includes:

£971m spent on Disability Living Allowance, which has surged by more than 22% in five years;
£179m on Jobseeker's Allowance - broadly unchanged from 2010 despite the number of people in work rising significantly;
£655m on housing benefit - a rise of 17% in the last five years;
And £734m on employment and support allowance, which has jumped by 39% in the last year.
In total, £5.583bn was spent on welfare in Northern Ireland during 2014/15, including £2bn on the retirement pension.

That total has risen by 15% since 2010/11, when expenditure added up to £4.851bn.