Dennis D. McCarthy, the Montgomery County dogcatcher who for 18 months refused to do his $25,573-a-year job because he said he was allergic to dogs, was suspended without pay yesterday and will probably be fired, officials said.

McCarthy's boss during the 18 months, county animal control director David E. Wheeler, also was removed from his post and likely will be reassigned to another county department, officials said.

Since March 1982, McCarthy, 61, had spent his work days reading books and listening to classical music rather than catching dogs--a task he said gave him high blood pressure, a skin rash and emotional stress. McCarthy also claimed he was illegally placed in the dogcatcher's job after he was transferred to the animal control unit in January 1980.

Yesterday, the 11-year county employe remained unrepentant.

"I'm happy as hell they fired me," he said. "I'm lucky I had something to engage and excite me all this time. I would have gone out of my mind if I'd liked popular music or couldn't read."

McCarthy was in fact reading "Exocet," a new Jack Higgins thriller, when he was summoned to a meeting yesterday afternoon with Thomas B. Ferguson, a county management specialist and now the acting head of the animal control department. It was Ferguson who earlier this summer alerted top administration officials to McCarthy's job performance.

At the meeting, according to both men, Ferguson ordered McCarthy to start performing his job. McCarthy refused and was given a suspension notice citing him for failing to perform his duties "in a competent or acceptable manner" and for "insubordinate behavior by failure to obey lawful directions" of a supervisor.

The notice said McCarthy's refusal to work had resulted in "payment of salary for non-productive work, reduced service to the citizens of this county, poor morale among department employes and embarrassment and loss of credibility" in the animal control program.

McCarthy indicated that he will not fight for his old job. "What's the sense of appealing?" he said. "That would be a chore. And I'm fast running out of energy."

Ferguson, noting that two recent medical examinations showed McCarthy has no allergies to dogs or other animals, said he will probably order the dogcatcher's dismissal next week. McCarthy could appeal the firing to the county's merit system protection board.

Wheeler, meanwhile, is expected to assume a new post in the environmental protection department, administration sources said. Wheeler said he expects to receive a "considerable" pay cut from the $50,340 he earned as animal control director.

"I made several efforts to get him [McCarthy] to work," Wheeler said, adding that McCarthy's time-consuming employment grievances against the county further hampered his efforts. "But basically, it was my fault because I couldn't figure out how to deal with the problem."