Solicitor T. Timothy Ryan will say his farewells this week as he resumes private law practice at the Washington firm of Pierson, Ball and Dowd. He said he accomplished his main goal--to "get hold of the mess" in the Teamsters Central States Pension Fund.

"That $6.5 billion is now in good hands," he said.

The department had charged former trustees with mismanagement of the scandal-ridden operation. Last year, the fund and the department signed a consent decree that requires, among other things, new independent management for the fund.

Ryan is leaving for personal reasons: "Money, all the reasons that drive anybody out of government," he said, adding that he had never meant to stay longer than two years.

He leaves this parting wisdom: "It's time for any person to leave government when you get used to having an enormous office with 15-foot ceilings and a black limousine with a phone in it."

Ryan's deputy, Frank Lilly, becomes solicitor and, according to Ryan, has the "inside shot" to replace him permanently.

CLEARING THE AIR . . . The dark clouds trailing Secretary Raymond J. Donovan have been scattered a bit by the recent indictment of an FBI informer on charges that he lied to federal agents about alleged ties between Donovan and organized crime figures.

Said one veteran at the department: "That really cast more doubt on the whole range of charges" against Donovan. "I get that feeling talking to the career people here. He's not a guy under siege anymore, the way he once was."

Even before the indictment of his accuser, Donovan had launched a campaign to become a visible, fully functioning Cabinet officer and ward off efforts by some presidential advisers to oust him as a political liability.

This week, his aides circulated a letter to President Reagan from Frank Hanley, secretary-treasurer of the operating engineers union, noting the indictment of the informer and adding, "You are to be congratulated for the position that you took in this whole affair. While others quickly accused the secretary without any facts, you stood solidly behind him."

As reported in The Post, Michael Kepfler, 42, a trucking company executive, was indicted in Syracuse, N.Y., for repeatedly making false statements to FBI investigators during Donovan's Senate confirmation hearings and again during an investigation by special prosecutor Leon Silverman. Last September Silverman concluded that there was not enough evidence to support any charges against Donovan.

RULES, RULES, RULES . . . The department published its semiannual agenda of regulations in the April 25 Federal Register, with tidbits on everything from child labor to construction workers in Guam.

Among other things, the Labor-Management Services Administration is "looking into" the possibility of requiring labor organizations to include in their annual financial reports a breakdown of how their money was allocated to various activities, such as political contributions. Several court cases are pending, based on complaints of non-union workers who, in some states, are forced to pay union dues but don't support the causes the union dues are supporting.

The department also has proposed easing the requirements for distributing pension and welfare plan descriptions to workers, with an estimated savings of $215 million (and 11 million hours of paper work) to businesses over two years.

JOBS TO WATCH . . . The department has added more than 275 new occupations to its Dictionary of Occupational Titles. Some of the newcomers: solar-energy systems designer, nuclear decontamination research specialist, radiopharmicist and laserist. CAPTION: Picture, T. TIMOTHY RYAN . . . union's $6.5 billion pension fund "is in good hands"