With a dictionary. ;*) “Felt” as in the feeling is the past tense of the verb “feel” (e.g. mhothaigh), and “felt” as in the fabric is either a noun (feilt ) or the present tense of the verb “felt” (e.g. feiltíonn).
Given the lack of any context, two possible valid translations exist for this exercise, so both mhothaigh siad and feiltíonn siad should be accepted.
Use the present: "mothaíonn siad" (they "usually" feel)
They are feeling: "tá siad ag mothú"
They were feeling: "bhí siad ag mothú"
They will feel: "mothóidh siad"
They used to feel: mhothaídís or mhothaíodh siad
They would feel: mhothóidís or mhothódh siad
It is felt: mothaítear
It was felt: mothaíodh
See http://www.teanglann.ie/en/gram/mothaigh for all the grammatical forms of any Irish word