By social context
https://www.irishstatutebook.ie/eli/2010/act/24/enacted/en/html
The 2010 act ^ provided homosexual couples legal recognition of a civil partnership
They could not marry because the consitution protects marraige as a social good between a man and a woman.
A constitutional right is stronger than a civil partnership legislation right
The government also "snuck in" common law / notmarried rights to property so a Irish woman living in her partners home can sue for a % if the relationship breaks down. This will still be a suprise to a lot of unmarried couples.
Anyway the right to marry needed a change to the consitution and the population was focused on equal marraige rights
Ireland had a prominent "male and openly gay" politicians but not many lesbians so for voters the man wanting to have a female birth cert was "kind of a gay thing anyway"
https://www.irishtimes.com/news/politics/ireland-becomes-first-country-to-approve-same-sex-marriage-by-popular-vote-1.2223646
The vote passed and the Dail prepared a bill on changing all the legislation to include F/F M/M relationships.
https://www.oireachtas.ie/en/bills/bill/2015/78/
That was in September
https://www.irishstatutebook.ie/eli/isbc/2015_35.html
Commencement Date Whole Act 16 November 2015
The GRA was a Dec 2014 bill and voted on in July 7 months later
https://www.oireachtas.ie/en/bills/bill/2014/116/
The original bill (listed as a pdf in the link and the various changes) had to include provisions for marrige as the vote on this may not have been passed and they stayed as the marraige act had not passed yet.
https://www.irishstatutebook.ie/eli/2015/act/25/enacted/en/print
the sections for marraige were never signed into law.
Commencement Date
S. 9(2)(a) 4 September 2015
S. 9(2)(b) Not yet commenced.
S. 9(2)(c) 4 September 2015
S. 10(1) (a)-(e) 4 September 2015
S. 10(1)(f)(i) Not yet commenced.
So if I am reading the act correctly if the consitution did not change (so no same sex marraige) and the sections were signed as active legislation a male could get a GRC to become a legal female and then only marry a male
Effect of gender recognition certificate generally
18.
(3) A person issued with a gender recognition certificate may only—
(a) marry a person of the opposite gender to the preferred gender and reference in section 2(2)(e) of the Act of 2004 to “same sex” includes a reference to the same sex as the preferred gender,
(b) be a party to a civil partnership registration with a person of the same gender as the preferred gender and reference in section 2(2A)(e) of the Act of 2004, to “not of the same sex” includes a reference to not of the same sex as the preferred gender.