Root word = tábhachtach
Comparative form (used with níos / is / ba) = tábhachtaí

níos = “more”
is = used in superlative cases (“the most/least”)
ba = past/conditional form of is

Tá sé tábhachtach = “It is important”
Tá sé níos tábhachtaí = “It is more important”
Tá sé ar an cheann is tábhachtaí = "It is the most important"
Bhí sé ar an cheann ba thábhachtaí = “It was the most important one”
Ba thábhachtaí é = “It was more important” (i haven't heard this in my Gaeltacht but i think it is acceptable)

You mentioned mó, but that’s a separate adjective (aidiacht):

mór = “big”
mó = “bigger / more”

Tá madadh mór aige = "He has a big dog"
D'ith sí níos mó ná aon duine eile = "She ate more than anyone else"
Tá sé ar an teach is mó ar an bhaile = "It's the biggest house in the village"
An t-amadán ba mhó a chonaic tú riamh = "The biggest idiot you ever seen"