There isn’t a single, universally worshipped Celtic mother goddess, or even a single universally worshipped Irish mother goddess, by which I mean a female deity or divine figure associated with nurture and fertility exclusively.
Yes, there are Celtic mother-goddesses, often identified as matronae or matres. Notice the plural; there are many of them, and they appear to be closely tied to local geographic features, thus making them a bit different from mother-goddesses in some other mythologies. Powell points out that “These Matres, or Matronae, are usually depicted as three figures bearing symbols of fecundity. Amongst epithets, they sometimes possess locality names, demonstrating again their identity with the territorial nature goddess” (155-56).
Matrona (a Gaulish or Continental figure) is closely associated with the river Marne in France (Mac Cana 31). And there is a Welsh figure, Modron, who is the divine mother of the divine son Mabon, mentioned prominently in Culwch ac Olwen.
There are references all over the continent to Deae Matres, and (most especially in the Rhineland and Cisalpine Gaul) the Matronae. The iconography associated with these figures includes but is not limited to, long flowing robes, sometimes showing a breast, often accompanied by babies, young children, fruit, bread, grain, and the like (Green 1992, 155).
There are, for instance, the Matres Comedovae at Aix-les-Bains, and the Matres Domesticae venerated in Chicester, England, the Matres Griselicae of Greouls in southern Gaul, the Matres Nemausicae, the Matronae Aufaniea from the Rhine area, the Matronae Vacallinehae, and a host of other local based female fertility-related female deities, often associated with children and often depicted in threes (Green 1992, 146-47). You will also note that many of these (the Matres Comedovae, Matres Griselicae, and the Nemausicae) were closely associated with local springs (Green 1992, 160).
These goddesses are often triple, not in the Gravesian sense of Mother, Maiden, Crone (though some of the figures appear to be two younger or middle aged figures and one older female), but in the sense that the iconographic representations show three female figures, often virtually identical. Green 1992 mentions the images along Hadrian’s Wall in Britain, and includes a photograph of the triple goddess with a “baby, a napkin, and bathing equipment” from Vertault in Burgundy (155). Green also appears to unilaterally conflate the Matraes/Matronae/Modron figures with sovereignty figures, something that makes me a bit leery (Green 1992, 156). However, I do think there are very strong similarities between Modron and Mabon/Rhiannon and Pryderi and of course there is that triad which describes Branwen as one of the three great ancestresses of Britain. I also think that given the large area involved and the huge numbers of female deities, we need to be cautious about overt generalizations.
If this topic appeals to you, it might be interesting to peruse Green 1995 pp 105-16 where she offers a reasonably good summary of Celtic mother-goddesses in general, with footnotes to recent scholarship, though she does make the occasional unsubstantiated assertion. As usual though, Green and Thames and Hudson have done a superb job of supplying good quality photos.
There are many references to mothers in Irish myth; one that comes to mind quickly is Macha. But, like Rhiannon in Welsh myth, or Modron, it is risky to label Macha as exclusively a “mother goddess.” To do so is to fall into the mistake made by Caesar and an unfortunately large number of scholars today, of buying into the interpretatio romana in an attempt to reduce the abundance and wide variety of Celtic deities, from a variety of geographic areas and historical eras, to a convenient, neatly organized and pigeon-holed schema or pantheon based on the familiar deities of Rome.
Green, Miranda. Dictionary of Celtic Myth and Legend. New York: Thames and Hudson, 1992.
Green, Miranda. Celtic Goddesses. New York: Thames and Hudson, 1995.
Mac Cana, Proinsias. Celtic Mythology. New York: Barnes and Nobles Books, 1996.
Powell, T. G. E. The Celts. New York: Thames and Hudson, 1958; second ed. 1985; reprinted 1991.