The incident happened on Wednesday morning at the offices of Farren-Roarty Auditors and Accountants in Letterkenny.
Staff arrived for work at the premises on the Thorn Road on Wednesday to find the gates of the premises locked.
A Garda patrol car was at the scene of the incident and staff were told to go home.
Gardai are remaining tight-lipped about the reason behind the fire but it is understood to be malicious.
It is understood to be the second such attack on the premises in recent months.
Nobody was hurt in either attack which is believed to have happened around 3am.
Workmen as well as partner Seamus Farren were at the scene yesterday afternoon.
A number of windows in the premises have been boarded up.
The company has been acting as a receiver for a number of cases taken by various banks on behalf of the Revenue Commissioners.
A Garda spokesman would not be drawn on possible reasons for the attack.