Mr Maguire, 26, from Longwood, Co Meath, died when the Citroen C2R2 he was co-driving, went off the road during the Topaz Donegal International Rally on Saturday afternoon.
The service was held at the Mount Errigal Hotel in Letterkenny – rally headquarters.
Prayers were also said for the car driver, Shane Buckley, who was injured in the crash that occurred during the Knockalla Stage south of Port Salon.
Mr Buckley is being treated in Letterkenny General Hospital where his condition is described as stable.
Those attending the service were told that both men knew the dangers involved in motor sport but loved it nonetheless.
It appears that their car left the road at a right-hand bend – after it collided with a bank, it then hit the side of a ravine before coming to rest some 300 metres down an embankment.
The rally – one of the biggest sporting events in the country, will be reviewed in the light of the tragedy but it is expected to remain on the sporting calendar.
President of Motorsport Ireland, Joe Corcoran, who expressed his sympathy to the Maguire family, stressed that high standards were maintained within the sport.
The rally’s emergency services and the rescue services were on the scene of Saturday’s accident within minutes.
Shortly after the accident occurred Donegal Motor Club announced that the remainder of this year’s rally was being cancelled as a mark of respect to the Maguire family.
Clerk of the course, Danny Gormley, said their thoughts and prayers were with the Maguire family and the family of the competitor who was injured.
“Thomas loved rallying and his loss has deeply affected everyone involved in the rally. His loss would be felt by rallying fans all over the country,” he said
The annual rally is in its 38th year and was expected to attract an estimated 70,000 spectators to Co Donegal over three days.
It is the first time a competitor has been killed in the rally. In 2008, a spectator was killed and in 2002 two stewards were also killed.
Meanwhile, Gardaí at Listowel, Co Kerry are investigating the death of a young motorcyclist on Saturday night.
The accident, in which a tractor was involved happened 5km from Listowel on the road between the town and Tralee.
The motorcyclist, who was named locally as John Fuller, 24, of Kilfaney, Listowel, was fatally injured and pronounced dead at the scene. No other people were injured.
Gardaí closed off the N69 roadway after the accident.