You can use the plural form after all numbers whose unit is 3, 4, 5, 6, 7, 8, 9 and 10.
eg.:
aon charr amhàin
dhà charr
trì charr / carrannaì
ceithre charr / carrannaì
cùig charr / carrannaì
sè charr / carrannaì
seacht gcarr / gcarrannaì
ocht gcarr / gcarrannaì
naoi gcarr / gcarrannaì
deich gcarr / gcarrannaì
dhà charr dèag
trì charr/ carrannaì dèag…