5. Is there a ban on donations from Trade Unions to political parties?

Electoral Act 1997 Section 26 (1) (as amended by the Electoral (Amendment) (Political Funding) Act 2012 Section 17):  There shall be included in—(a) the report by the directors of a company under section 158 of the Companies Act, 1963 , and the annual return under section 125 or 126, as may be appropriate, of that Act, (b) the annual return to be made to the Registrar of Friendly Societies by a trade union (within the meaning of the Trade Union Acts, 1871 to 1990) or a society registered under the Industrial and Provident Societies Acts, 1893 to 1978, or the Friendly Societies Acts, 1896 to 1977, and (c) the report of a building society (within the meaning of the Building Societies Act, 1989 ) under section 78 of that Act, particulars of all donations (within the meaning of section 22 or 46 or regulations made under section 72 ) exceeding €200 in value made by the company, trade union, society or building society, as the case may be, in the year to which the report or return relates, including particulars sufficient to identify— (a) the value of each such donation, and (b) the person to whom the donation was made. (2) For the purposes of this section—(a) “company” has the same meaning as in the Companies Acts, 1963 to 1990; (b) all donations made by the company, trade union, society or building society, as the case may be, to the same person in the year to which the report or return relates shall be aggregated and treated as a single donation; (c) in the case of a contribution of the kind referred to in subsection (2)(a)(vi) of section 22 or subsection (2)(a)(vi) of section 46 regard shall be had to the gross value thereof.

There is no ban.
See Standards in Public Office Commission. 2015. Guidelines for Political Parties Steps to be taken concerning donations and prohibited donations,  p. 5.