Whenever someone willfully chooses to sacrifice themselves for a greater cause, they are honoring Lugh.
Common misconceptions: Lugh is often seen as a sun deity. There is no evidence for this. The sun is female in all Celtic cultures and Lugh is associated with light, but never the sun. He is not the Helios you're looking for.
Spirit of death and sexuality. Morríghan is a goddess in her own right, but she also appears as na Trí Morríghna, the Three Great Queens, given as separate goddesses below.
Causes: No charity work particularly represents Morríghan. You could imagine she'd support honoring the dead, but that's not her gig—she's Death itself, she is hungry and she seldom appears in a compassionate guise. Although any cause that sends people to die could be considered a toast to Morríghan, in a sense.
(Many people would attach feminist causes to her, but I'd leave that for Macha, below.)
Common misconceptions: Morríghan is often depicted as evil. But all the Irish gods are seen as embodying virtue. Morríghan and her aspects represent both a necessary natural force, as well as an ethical virtue: aggression against one's enemies.
Spirit of Victory. More accurately, the spirit of bua—a concept that involves the synergy of ethical virtue, inner and outer strength, and victory. The spirit of sovereignty, which she can bestow or revoke. Notably, she gave her life in the battle against the giants ("Macha the daughter of Ernmas fell at the hands of Balor...") and the very next passage shows Lugh turning the tide of the battle.
Causes: Pregnant women! Causes promoting wine, mead or alcohol. Horse-related causes. Preserving the monuments of Ulster/Northern Ireland. Political causes. Wars. Feminist causes.
An Badhbh Catha, a.k.a. "the Badhbh"
The Battle Crow, spirit of bloodshed and frenzy. There is a specific trance that sometimes comes over a warrior as they kill, so that they act with almost unconscious fury and grace—that's her.
Causes: Other than directly supporting soldiers' ability to kill people efficiently, I can't think of causes that honors her. Care packages for troops or morale-raising events are definitely not her thing.
Possible exception: causes that protect crows or ravens.
Spirit of abject terror, and also spirit of the sacred. Nemhain is a complex goddess who underscores some of the most rugged truths about transcendent practices.
Causes: Retreats and retreat centers. Supporting intensive, dedicated spiritual practice.
Manannán mac Lir
Spirit of old age and crossing boundaries. Patron of mariners and travelers.
Causes: Travel-related projects. Advancing longevity. Oral history projects to record the stories of seniors. Honoring elders. Boat-related causes. Causes related to funerals and honoring the dead. Causes that help orphans and foster children. Anything that supports or promotes the Isle of Man, the Manx language or the Manx people.
Common misconceptions: Often mistakenly said to be the god of the sea. The sea is actually a fomhor (titan) named Tethra. There are many parallels between the two but Tethra appears to be the sea itself in all its might and fury while mac Lir is the god who traverses the sea between the worlds. Another deity, Nechtan, could be viewed as the god of the sea but is more directly associated with wells. Nechtan is identical to Nuadu, given below.
A suggested reconstruction names Aine as the spirit of the sun. I feel the evidence in the lore is strong for this. She is also associated with flowers and the fertility of crops.
Causes: Agricultural causes. Curing breast cancer. Solar power. Youth development projects aimed specifically at girls/young women. Preserving the monuments and heritage of Munster.
Spirit of life-giving fire on earth. Does not rhyme with "rigid"—pronounced more like "Bree-ij."
Causes: Helping mothers. Helping children. Cooking-related causes. Home-related causes. Free clinics. (A medical cause that charges the patient money, even at a deep discount, is not a good fit for Brighid.) Veterinary causes and caring for animals. Dairy-related causes. Bringing hope to people. Forgiveness. Interfaith work. Shelters for battered women. Creating safe shelters or refuges of any kind. Aiding refugees. Shielding others.
Alternate: Brighid's son Ruadhan betrayed the gods and was speared through the back as he fled to the giants. Brighid was the first mother ever to keen. In this aspect, causes related to grief would also be appropriate.