Ireland Women's hooker Gillian Bourke scored a late try to deny their Welsh counterparts at Aberavon's Talbot Athletic Ground.
The visitors struck first through lock Sophie Spence's try, converted by Niamh Briggs.
Rosie Fletcher replied for the hosts, Elinor Snowsill adding the extra points and Laura Prosser put the hosts in front with a 69th-minute penalty.
But Bourke went over five minutes from the end to seal Ireland's win.
Wales Women: Rosie Fletcher (Neath Athletic); Caryl James (Cardiff Quins), Elen Evans (Waterloo), Rebecca de Filippo (Cardiff Quins), Philippa Tuttiet (Cardiff Quins); Elinor Snowsill (Bristol), Amy Day (Unattached); Megan York (Blaenau Gwent), Lowri Harries (Neath Athletic), Catrin Edwards (Cardiff Quins), Gemma Hallett (Pontyclun), Shona Powell Hughes (Neath Athletic), Catrina Nicholas (Cardiff Quins), Sioned Harries (Cardiff Quins), Rachel Taylor (Bristol, capt).
Replacements: Carys Phillips (Cardiff Met), Bethan Howell (Seven Sisters), Jenny Davies (Waterloo), Nia Davies (Cardiff Met), Vicky Owens (Pontyclun), Laura Prosser (Pontyclun), Dyddgu Hywel (Caernarfon), Rafiuke Taylor (Sea View).
Ireland Women: Niamh Briggs; Niamh Kavanagh, Grace Davitt, Jenny Murphy, Alison Miller; Lynne Cantwell, Lariss Muldoon; Fiona Coghlan, Gillian Bourke Ailis Egan, Sophie Spence, Marie Louise Reilly, Laura Guest, Claire Molloy, Joy Neville.
Replacements: Stacey-Lee Kennedy, Fiona Hayes, Lauren Day, Heather O'Brien, Siobhan Fleming, Amy Davis, Nora Stapleton, Ashleigh Baxter.