Catalyzes the demethylation of isoprenylcysteine methylesters. In vitro, is specific for N-acetyl-S-farnesyl-L-cysteine methyl ester (AFCme) and has low activity toward N-acetyl-S-geranyl-L-cysteine methyl ester (AGCme). Acts as a positive regulator of ABA signaling. May be involved in the demethylation and inactivation of isoprenylated negative regulators of abscisic acid (ABA) signaling. Carboxyl methylation is a reversible and potentially regulated step in the post-translational modification of prenylated proteins. [protein]-C-terminal S-[(2E,6E)-farnesyl]-L-cysteine methyl ester + H2O = [protein]-C-terminal S-[(2E,6E)-farnesyl]-L-cysteine + H(+) + methanol Expressed in roots, rosette and cauline leaves, stems, flowers and siliques. By ABA, osmotic stress and salt and heat treatments. Plants overexpressing ICME exhibit enhanced ABA inhibition of seed germination and ABA induction of stomatal closure. Plant silencing ICME exhibit a reduced ABA sensitivity in seed germination assays. Belongs to the AB hydrolase superfamily. Isoprenylcysteine methylesterase family.