2. Is there a ban on donations from foreign interests to candidates?

Electoral Act 1997 23 A (2) (as amended by Electoral (Amendment) Act 2001: None of the persons referred to in any of paragraphs (a) to (f) of subsection (1) shall, directly or through any intermediary, accept a donation of whatever value given by—(a) an individual (other than an Irish citizen) who resides outside the island of Ireland, or (b) a body corporate or unincorporated body of persons which does not keep an office in the island of Ireland, being an office from which the carrying on of one or more of its principal activities is directed.

There is a ban.
See Standards in Public Office Commission. 2017. Guidelines for Members of the Houses of the Oireachtas and Representatives in the European Parliament relating to acceptance, management and disclosure of donations, p. 16.