Whatever their policy in the north before the referendum result in the south, describing Sinn Fein as being 'as opposed to abortion as the DUP' seems like a bit of an exageration.
Back in 1985 a Sinn Fein Ard-Fheis (National Conference) passed a motion adopting a pro-choice policy. You can see some of the backlash within the party at the next year's Ard-Fheis
here from page 45 onwards, although there's also a motion from a Cumann (branch) from Derry trying to maintain a pro-choice line. While this might not do much for the party's feminist credentials, the extent to which the issue was being contested within the party was well ahead of other major political parties either side of the border.
Sinn Fein's position on women's reproductive rights has historically reflected the social conservatism prevalent in Irish society, north and south, in combination with other tendencies with an more progressive attitude to women's rights at least nominally, even if they were prepared to put these's issues on the back burner indeffinitely.
Edited to add:
This
article also includes a good critical overview of some of the tensions within Sinn Fein's position on reporductive rights.