Will something like claritin (loratadine) or zyrtec relieve cat allergy symptoms?

Possible: Oral antihistamines are used to treat itchy nose, eye and sneezing. So if one is experiencing these symptoms use of anti histamines would be reasonable.

Partially: The most effective medication treatment for cat allergy is nasal steroid sprays like Flonase for example. Nasal steroid sprays are more effective than antihistamines, but take longer to work (few days). If cat exposure is intermittent and anticipated, then Claritin or zyrtec before cat exposure may be helpful. Allergy shots are the best long term medical treatment.